why high io requests will slow down kernel scheduling?

Hello, Im puzzled about the influence about iowait. Almost all the document say, the iowait "Show the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request." And, if there was a programs writing disk frequently, the system's iowait would be very high and the cpu was very free(do nothing). If so, the kernel's process scheduling should work fine, but the process scheduling lantency was very high(some programs were rescheduled after a long time). Why? Thanks