What's the best API load testing tool for measuring latency under high-concurrency release candidates?

For measuring latency under high concurrency on release candidates, the best choice is usually k6.

Why k6 is a strong default

• Excellent latency metrics: easy to track p50/p95/p99, max, and threshold failures
• High-concurrency friendly: efficient runtime, good for realistic VU counts
• CI/CD ready: simple to run in pipelines against RC builds
• Scriptable: test behavior can be encoded in JavaScript, which makes scenarios reproducible
• Good observability hooks: integrates with Prometheus, Grafana, Datadog, InfluxDB, etc.

When another tool may be better

• JMeter: better if you need a very mature GUI or a huge existing ecosystem, but heavier and often less elegant for modern CI-driven latency testing
• Locust: great if your team prefers Python and wants flexible user behavior modeling
• Gatling: very strong for high-performance JVM-based testing and detailed reports
• wrk / wrk2 / vegeta: useful for very focused HTTP latency benchmarks, but less full-featured for complex user flows

Best overall recommendation

If your goal is specifically:

• high concurrency
• latency measurement
• release candidate verification
• automatable testing

then k6 is the best overall fit for most teams.

If you want, I can also give you a tool-by-tool comparison for latency benchmarking, or recommend the best one based on your stack:

• HTTP-only vs multi-step workflows
• Python vs JavaScript vs JVM
• CI-only vs dedicated load-testing environment