>Software rendering simply stinks, doubly so if you're running a fancy composited desktop. Regardless of CPU speed or UI fluidity, your CPU and GPU processes are now fighting for the same cycles, bottlenecking one another in the worst way possible.
And yet, after first release use, they have reported that this is the smoothest they've seen a Linux desktop ever run. That is, smoother even when compared to intel-Linux on hw GPU acceleration.
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.
And yet, after first release use, they have reported that this is the smoothest they've seen a Linux desktop ever run. That is, smoother even when compared to intel-Linux on hw GPU acceleration.