The point was that 15-30% is a very big speedup for a change that impacts what is supposed to take a very small part of the total time. See also Amdahl's law [1

The point was that 15-30% is a very big speedup for a change that impacts what is supposed to take a very small part of the total time. See also Amdahl's law [1] Suppose that the CPU-intensive part of the program was actually speed up by 3x. This means that previously it took 1.5x as much time as the one that was saved, so thoughly on the order of 22-45% of the total time of your program. In practice the speedup was probably lower than 3x, which means the 22-45% figure would be even higher. These are really really high number for a system that was supposed to spend most of the time (>90%) blocked in IO (which was not speed up by YJIT). For comparison, if the CPU-intensive part actually took 10% of the time, then 10% would be maximum speedup you could get (which is way lower than 15-30%) and a 3x speedup would only result in a ~6% speedup. [1]: https://en.m.wikipedia.org/wiki/Amdahl%27s_law https://en.m.wikipedia.org/wiki/Amdahl%27s_law