If you have a running resync job of mdadm this job can impact your normal operations a lot under different circumstances. You can limit the resync throughput with two kernel variables:
/proc/sys/dev/raid/speed_limit_min/proc/sys/dev/raid/speed_limit_max
You can set here the minimum and maximum bandwidth in kibibytes per second(1024 Bytes).
Unfortunately you cannot set 0 to pause the resync. But you can set it to 1 kibibyte per seconds. This is nearly zero and is enough to be nearly pause.
But pay attention! If you raid is not sync you risk data loss.