As someone else mentioned, test suites are a bit of a tough case for JITs. That said, we really don't want to slow down any workloads, and I suspect that something might be going wrong if you are getting a measurable slowdown.
It's not, but after reading the other answer here I can totally see why it might be slightly slower. I'll definitely do some more testing with it beyond just timing my test suite runs.
Is this project public? I'd love to investigate