Basically anything that might be scripted. While there are high frequency CPUs around, the trend is clearly towards multi-core, low frequency.
It really sucks to wait minutes for a medium sized script to finish while the CPU literally sits idle.
With Grasshopper and scripting being an integral part of Rhino now, I think the design factor for commands execution speed should be automation, not user interaction… And user experience would profit for heavier operations where users currently see a delay in execution.
I am aware not every operation lends itself to parallelization, but that’s the point. Find out what can be done and do so. And if it can’t be done, at least make it thread safe.