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My very unscientific test (just running the test suite on the Django project I happen to have up at the moment) proved to be very slightly slower with it installed: 11.2s - 11.3s without Pyston, 11.7s - 11.8s with Pyston.

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A test suite would be designed to call every routine in a project a limited number of times. This means the JIT compiler will be compiling each routine, for it to be run a few times then thrown away.

Realistic loads might have the same routines run more intensely and might see more benefit from the JIT compiler.

Ahhh, of course. Good point, thanks.

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.

Is this project public? I'd love to investigate

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.

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