I profiled a huge legacy tests collection using cProfile, and found lots of low hanging fruits. Like some tests were creating 4000x3000 Pillow image in memory just to test how image saving code works (checkign that filename and extension are correct). And hundreds of tests had created this huge image for every test (in the setUp method) because of unittest reliance on inheritance. Reducing size image to 10x5 made the test suite faster for like 5-7% (it was long time ago, so I don't remember exact statistics).
But the changes in TFA were of the other of 75% improvement for "dumb" changes that were agnostic to the details of the tests being run.
Saying you got a 5-7% improvement from a single change, discovered using the profiler, that took understanding of the test suite and the domain to establish it was OK, and that actually changed the functionality under test - that's all an argument for doing exactly the opposite of what you recommend.
> that actually changed the functionality under test - that's all an argument for doing exactly the opposite of what you recommend.
It was an old functionality. Someone wrote a super class that for the need of testing filesystem functionality created extremely large images. Not only there was no need to test with such large images, other developers eventually inherited more testcases from that setup code (because there were other utility methods), and now setUp code was needlessly creating images that no test used.
Generating a huge 4k image takes a significant time using Pillow.
So, I'd run the tests under cProfile first.