Build performance has been a pet topic for me for quite some time when I realized I was wasting so much times waiting for stuff to build 14 years ago. The problem is especially endemic in the Java world. But also in the backend world in general. I've seen people do integration tests where 99% of the time is spend creating and recreating the same database over and over again (some shitty ruby project more than a decade ago). That took something like 10 minutes.
With Kotlin/Spring Boot, compilation is annoyingly slow. That's what you get with modern languages and rich syntax. Apparently the Rust compiler isn't a speed daemon either. But tests are something that's under your control. Unit tests should be done in seconds/milliseconds. Integration tests are where you can make huge gains if you are a bit smart.
Most integration tests are not thread safe and make assumptions about running against an empty database. Which if you think about it, is exactly how no user except your first user will ever use your system.
The fix for this is 1) allow no cleanup between tests 2) randomize data so there are no test collisions between tests and 3) use multiple threads/processes to run your tests to 1 database that is provisioned before the tests and deleted after all tests.
I have a fast mac book pro that runs our hundreds of spring integration tests (proper end to end API tests with redis, db, elasticsearch and no fakes/stubs) in under 40 seconds. It kind of doubles as a robustness and performance test. It's fast enough that I have codex just trigger that on principle after every change it makes.
There's a bit more to it of course (e.g. polling rather than sleeping for assertions, using timeouts on things that are eventually happening, etc.). But once you have set this up once, you'll never want to deal with sequentially running integration tests again. Having to run those over and over again just sucks the joy out of life.
And with agentic coding tools having fast feedback loops is more critical than ever.