I've read that the shielding is a small part of it (enviro temperature fluctuation can be hundreds of degrees) but the biggest factor is the process is around 6

I've read that the shielding is a small part of it (enviro temperature fluctuation can be hundreds of degrees) but the biggest factor is the process is around 65nm or much, much larger to help prevent radiation from accidentally toggling gates. They are very constrained in terms of available power - and under-clocked if you wish. The radios I believe consume most of the power. The multiple redundancy / failover nature of every circuit including RAM, CPU stack/heap, etc. also bites into the power budget which slows things down. On some space and extraterrestrial vehicles just about everything is double-computed on separate circuits and compared for exactness, and can be compared with results on an earth-based system for accuracy. I'm sure some readers here can explain some of these features in more depth.