the usual speedup with sse/avx is in about that 2.5 range from what i've observed. If it's lower, then there are likely other effects that stall the execution,

the usual speedup with sse/avx is in about that 2.5 range from what i've observed. If it's lower, then there are likely other effects that stall the execution, like cache misses and other data logistic ineffieciency. The fft shown here uses a precalced lookup table for sin & cos. For a fft size of 2048 this takes 2 * 8 * 2048 bytes, around 32 kb, which is the already size of the usual L1 cache. The bigger the fft size the bigger the lookup table and the less all the asm will be effective. The asm benefits more of a pure algorithmic approach, might try some sin cos approximation instead of a lookup table, then the speed gain might be more consistent.