In general AVX and more cores only helps slightly… remember CPU memory bandwidth has not went up much in recent years, all you have is about 50GB/s or so in an ideal case, and even with added cores, it isn’t enough for graphics rendering, even the slower GPUs thave way over 100GB/s and a reasonably usable GPU is going to have 200+GB/s and a high end one 1TB/s
And on top of that a CPUs cache is not designed for graphics work.
Investing effort into CPU fallback support is almost a fools errand. It should be there yes, but don’t expect it to actually be usable for real work or anything more than 10+ year old games. It’s great to see things like Blender load… and maybe even do simple work in it… but you can’t expect it to perform.
Even AMD’s on package GPU’s top aout at around 1.5TF/s due to memory constraints… all the bandwidth is used up at that point. And that is with a GPU optimized cache in front of the memory controller.