Both tools load-test web applications. They go about it very differently. Here's where each one fits, written as fairly as we can manage.
Evaluat is a real-browser performance testing platform that runs each virtual user in an isolated browser instance, capturing Core Web Vitals and Navigation Timing metrics under load — with session video, network logs, and console logs for every user.
k6 is an open-source HTTP-level load testing tool from Grafana Labs, scriptable in JavaScript, with a managed cloud product (Grafana Cloud k6). It sends raw HTTP, gRPC, WebSocket, and other protocol requests and measures server response.
The categorical difference: k6 simulates users at the HTTP request layer. Evaluat simulates users in real browsers. k6 measures server response time. Evaluat measures what the user actually sees, Web Vitals included.
| Capability | Evaluat | k6 |
|---|---|---|
| Real browsers per virtual user | ||
| Core Web Vitals (LCP, INP, CLS, FCP) | ||
| JavaScript execution under load | ||
| Third-party tag impact measurement | ||
| Per-session video recording | ||
| Full network logs per session | Partial (HTTP only) | |
| Console logs per session | ||
| Step-by-step playback | ||
| HTTP-level load testing | ||
| gRPC, WebSocket, MQTT protocols | ||
| API contract testing | ||
| Open source | ||
| Scriptable in JavaScript | Visual scenarios |
You're load-testing an API, not an application. If the unit of work is a REST or gRPC request, the user experience isn't a meaningful concept. k6 is purpose-built for this. It will drive tens of thousands of concurrent virtual users on a single machine without breaking a sweat.
You need very high concurrency on a tight budget. Real browsers are expensive to run. HTTP requests are cheap. If the test you need is "can we survive a million concurrent users hammering the homepage" and you don't care about Web Vitals, k6 wins on cost by an order of magnitude.
You already have a JavaScript-heavy testing stack and your engineers prefer code. k6's scripting API is good. Tests live in version control alongside your code. If that workflow matters to you more than the visual scenario builder, k6 is the better fit.
You care whether the site stays fast for real users. Server response time is not the same thing as page load time. A 50ms TTFB doesn't help if the LCP image takes 4 seconds because three third-party tags are blocking the main thread. An HTTP-level test can't see that. A browser can.
Third-party scripts are a meaningful part of your page weight. Analytics, A/B testing, consent banners, chat widgets, marketing tags — these run in browsers, not on your servers. They have real impact on INP and LCP. Evaluat measures them by definition. k6 can't.
You need to debug what actually happened. When a load test fails, the question is always "for which users, on which steps, with what symptoms?" Evaluat keeps per-session video, console output, network waterfall, and step-level pass/fail. You watch what happened. One-hour root cause instead of one-week.
Your stakeholders speak in Core Web Vitals. If your engineering KPIs are LCP, INP, and CLS — which they probably should be for any user-facing application in 2026 — those are the numbers your performance testing tool needs to produce.
k6 runs from load generators that fire HTTP requests directly. No HTML parsing, no JavaScript execution, no rendering. Its outputs are request-level: response time, throughput, error rate, status codes. Fast, cheap, accurate for what it measures.
Evaluat runs each virtual user inside a real, isolated browser. The browser parses HTML, executes JavaScript, fetches sub-resources, runs your third-party tags, paints frames, and reports Web Vitals exactly like a real user's browser would. Main-thread blocking, layout shifts, interaction latency — all measured authentically.
You can't approximate one with the other. They answer different questions.
Keep k6 for the things it does well. API load tests. Backend stress tests. Protocol-level work. The stuff that doesn't need a browser.
Use Evaluat for the customer journeys. Login, search, checkout, signup, account dashboard. The journeys where the metric that matters is the user's experience, not the server's response time. Most teams end up with k6 owning a small piece of the API surface and Evaluat owning the whole user-facing side.
For user-facing applications, yes. For API load testing or protocol-level work (gRPC, MQTT, raw WebSocket), no — Evaluat doesn't test at that layer. Most teams use both.
Per virtual user, yes. Running a real browser costs more than firing HTTP requests. But the unit of work is different too. A 1,000-user real-browser test produces information you can't get from a 1,000-user HTTP test at any price. The question is which information you actually need.
k6 has an experimental browser module based on Playwright. It works for small concurrency targets and basic Web Vitals capture. It's not the main thing k6 is built for, and it doesn't give you per-session video, full network log capture, or the kind of forensic report Evaluat produces.
Triggering tests from CI is on the roadmap as part of Testing Suite, which uses the same scenarios as your performance tests but runs them as deployment-gate smoke tests. Join the waitlist if that's the workflow you need.