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 assign

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.