You will not get enough CPU time given to a task if it does, say, 10% IO, and 90% CPU if there is an IO hog running in parallel. You either IOnice everything to oblivion, or bare with this.
IO blocks are almost always CPU blocks too on anything that does even minimal IO. And the more simultaneous IO, the more CPU overhead there is.
Try running compiling something big with -J 32 or more.
When system is IO busy, processes block (more) waiting for IO completion. IO blocked processes wouldn't know what to do with a CPU, so they won't have it assigned. Are you saying something different?
Decreasing dirty memory decreases flush times, decreasing process write/flush times, decreasing process/UI hangs/pauses. Presumably it also lowers throughput in some cases, but it's better than having UI hangs.
IO blocks are almost always CPU blocks too on anything that does even minimal IO. And the more simultaneous IO, the more CPU overhead there is.
Try running compiling something big with -J 32 or more.