@JoeJ  two things told this away. first was, when i switched to a framebuffer code that was far simpler and far less cycles (this was aligned to 32 bit instead of 24 bit bitwise magic). i expected some speed gain from the simpler pixel code, but instead, the speed fell by 15-20% or something, …

For choosing what instruction sets to use, the steam hardware survey (https://store.steampowered.com/hwsurvey/) is helpful - look in the "other settings" section. For example, it will tell you that about 10% of Steam users don't have AVX capable CPUs.