Every product that depends on voice input lives or dies by its speech-to-text performance. Whether you're enriching CRM data from support calls, powering live captions in meetings, or triggering downstream actions via LLMs, transcription accuracy and speed aren’t just nice-to-haves. They’re essential to product functionality. If your STT engine stalls on latency or mistranscribes a customer’s request, it can break automations, derail user experiences, and create costly manual work downstream.
That’s why smart teams don’t treat STT as a one-time integration. They benchmark. Regularly. Rigorously. And with real-world audio that reflects how their users actually talk.
In this guide, we’ll walk through the core metrics to measure, common pitfalls to avoid, and best practices for stress-testing STT APIs.
Before you start comparing providers, it’s important to understand the numbers you’ll be looking at, and where they can mislead you. Most vendors advertise metrics like Word Error Rate (WER) or low latency, but raw scores often fail to capture what actually matters in production.
WER is the standard industry metric. It calculates how many words in a transcript differ from a reference transcript, using three key types of errors:
Here’s the standard WER formula:
While WER is simple and standardized, it doesn’t always reflect real-world performance. We’ll explore this in more detail shortly.
WAR (Word Accuracy Rate) is the flip side of WER. It’s often a more intuitive metric to use when talking to stakeholders because it communicates performance in positive terms ("94% accurate" instead of "6% error rate").
WER gives you a starting point, but it leaves out a lot of context that impacts real-world usability. From critical content errors to language normalization and speaker bias, here’s why relying on WER alone is risky.
In most applications, some words matter a lot more than others.
In healthcare, mistranscribing a dosage (“fifty” instead of “fifteen”) can lead to life-threatening outcomes. In finance, omitting a currency or decimal point (“two thousand” vs “twenty thousand”) can cause major compliance issues. And in customer support, missing an order number or customer name could have a direct impact on retention.
That’s why it’s important to look for an STT vendor that optimizes for high-value content — like names, numbers, and identifiers — not just overall averages.
Want to learn more about how to evaluate vendors? Check out our STT Buyer’s Guide.
One of the biggest misconceptions about WER is that it’s a purely objective score. In reality, it’s only as reliable as the normalization process used to calculate it.
WER normalization means standardizing both the reference transcript (the “ground truth”) and the machine-generated transcript into a standardized format before comparing them. This includes how things like punctuation, numerals, contractions, and filler words are treated.
The problem? Normalization rules vary between vendors, datasets, and teams. That makes WER results inconsistent and hard to compare.
Some examples:
Without consistent rules, two models might perform the same, but get very different WER scores.
For example:
This is especially problematic in multilingual or accent-heavy contexts, where spelling variants and contractions are more common. In these cases, small formatting differences can add up to large accuracy penalties.
WER doesn’t account for whether a model is working with full context (like an LLM has in batch or async processing) or only a narrow view of the conversation (like an LLM has in real-time streaming).
That’s because real-time systems must begin generating output within a few hundred milliseconds. This limited view means the model doesn't yet have the full context of the sentence, conversation, or speaker intent. As a result, it may misinterpret ambiguous phrases, misplace punctuation, or choose incorrect grammatical structures — especially in the case of similar-sounding words (like 'their' vs 'there') or incomplete phrases that rely on context to interpret correctly.
Async systems, on the other hand, process audio after the fact, with access to longer sequences and full conversations. The result? More accurate decisions on phrasing, punctuation, and word choice.
Because real-time systems generate output with limited context, their WER scores often appear worse than async systems, even if they're performing as expected. That’s why teams should avoid comparing WER across different processing modes and instead evaluate each system within its intended use case.
WER averages don’t reveal how well a model performs for different groups of people.
Many STT systems underperform for women, non-native speakers, and people of color because the training data lacks prosodic diversity. Males typically achieve over 90% accuracy, females around 80%, and children as low as 40%. Whisper in particular has known accuracy gaps tied to speaker demographics.
This matters: a model that nails the average but fails for key user groups can’t be considered truly accurate.
Accuracy isn’t just about getting the right words. It’s also about how they’re delivered.
For many products, especially in customer experience, formatting directly affects usability. A phone number transcribed as “six six five eight four nine four nine nine eight” instead of “665-849-4998” can’t be copied, clicked, or used to trigger an automation. Similarly, a missed capital letter in a name (“gladia” instead of “Gladia”) can look unprofessional in a support transcript or email follow-up.
Captions, summaries, and structured data extraction all require readable and reliable formatting to function correctly.
There are two latency metrics that matter, and they’re often conflated.
We believe latency to final is the more meaningful benchmark. TTFB can be artificially fast in controlled environments or for trivial audio snippets. But if your automation is waiting on final output, that’s what you need to measure.
To get a true sense of performance, make sure you:
We followed these exact best practices when benchmarking our newest model, Solaria. The result: Solaria consistently outperforms the competition on both TTFB (~270ms) and latency to final (~698ms).
Clean audio and perfect pronunciation are great for demos. But real-world applications are messy, and that’s where most transcription models fall apart.
Let's take a closer look at the variables that actually affect STT performance in production: background noise, accent diversity, and real-time speaker variation. These are the challenges most models struggle with, and the ones your benchmarks should reflect.
A customer is calling from a busy kitchen on speakerphone. Their dog’s barking, someone else is talking in the background, and the agent’s headset isn’t great either. This is the reality of contact center audio, and it’s brutal for most transcription models. These same acoustic challenges show up elsewhere, too. In sales calls, virtual meetings, and anywhere else voice interfaces are expected to perform reliably.
The environment a model was trained on directly impacts its performance.
Solaria wasn’t trained on cherry-picked samples. It was built on noisy, real-world, multi-speaker conversations...the kind you’d actually expect in production.
It’s easy to claim support for “100+ languages.” But does the model work equally well for an accented speaker from Bangalore or Dakar? Can it handle users switching between Spanish and English mid-sentence?
Many models degrade significantly outside of clean, US-accented English. Gladia supports 100 languages — including 42 underserved ones — and is specifically designed for accent robustness and real-time code-switching.
As we mentioned earlier, research has repeatedly shown that many STT models underperform for women, non-native speakers, and people of color. That's because many training datasets are gender-biased, age-biased, and lack prosodic diversity.
Gladia takes a deliberate approach to mitigating bias by using diverse, customer-approved data and real-world recordings in our training pipeline.
We’ve talked a lot about why benchmarking STT APIs is so complex. But what can teams do about it? Based on our work with hundreds of start-ups and enterprise companies, here are some tried and tested best practices.
Start with public datasets like Mozilla Common Voice and Google FLEURS. They offer accent and dialect diversity. But don’t stop there...
Use your own audio, too.
The real value comes from testing transcripts of your customer calls, your audio environments, and your domain-specific language. Also be sure to test across multiple versions of public datasets. Some vendors overfit to a single release.
Track not just overall WER or WAR, but accuracy on high-value content: names, numbers, locations, spelled-out emails.
Look at:
Run streaming tests and measure for both TTFB and how long it takes to return the finalized transcript (final latency). Both matter: one affects responsiveness, the other determines when downstream processes can start.
Also test at concurrency — running 50 or more simultaneous audio streams — to ensure performance scales. For startups, this prevents early architecture choices from becoming a bottleneck. For enterprise teams, it’s how you ensure reliability under production load and avoid unexpected latency spikes at scale.
Most APIs are configurable. For example, Gladia offers:
Take the time to experiment with these settings early. Tuning them to match your real-world inputs can dramatically improve accuracy and save you hours of post-processing down the line.
Models evolve. So do your users. A provider that performs well today may underperform next month after a model update or shift in audio mix.
Most of our customers benchmark on a quarterly basis. They track latency, WAR, and entity accuracy over time to help them catch regressions early and ensure consistent performance as their audio environments evolve.
When we built Solaria, we weren’t optimizing for leaderboard scores. We were optimizing for reality. From noisy call centers to multilingual conversations with overlapping speakers, Gladia is designed to perform in the conditions that actually challenge most speech-to-text systems.
Gladia delivers:
Unlike providers who only test on clean public datasets, we validate performance across:
Our benchmarks reflect how STT performs in production — not just in ideal lab conditions. That’s how we ensure our customers get consistent, reliable results.
Want to see how Gladia performs on your own audio?
Start testing today with free credits and dedicated 1:1 support from our team. We’ll help you run benchmarks, interpret results, and fine-tune the model to your needs so you can ship faster, with confidence.
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