I use llvmpipe currently and I can assure you that it is no where near as smooth as a setup with hardware acceleration. The 3900x is probably even faster than the M1 at software rendering and it still isn't fast enough to give a consistent 60fps with the browser using most of the screen (shrinking the browser window makes things much more smooth).
Even extremely fast CPUs suck really bad at pushing pixels compared to even the weakest GPUs. It is very much usable though!
Currently llvmpipe is able to use up to 8 cores at a time but not more, and does use SIMD instructions when available from my understanding. There is another software rendering system in MESA that allegedly uses AVX instructions but I have had a better experience with llvmpipe personally.
Even extremely fast CPUs suck really bad at pushing pixels compared to even the weakest GPUs. It is very much usable though!
Currently llvmpipe is able to use up to 8 cores at a time but not more, and does use SIMD instructions when available from my understanding. There is another software rendering system in MESA that allegedly uses AVX instructions but I have had a better experience with llvmpipe personally.