Adding hd slow

What matters most for the resync time (apart from the slowest disk speed and other hardware such as CPU) is how actively the NAS is being used. The disks have to be synced sector by sector regardless of how much data is on it. Obviously if resources are being used for other things that's less that's available for doing the resync.

If you want to minimise rebuild time turning off services you don't need to run during this time may help. You can turn them on again afterwards.

If you don't need Plex perhaps turn that off and turn it back on just for the period you need it. Or services like the iTunes server or ReadyDLNA.

If you are an advanced user who likes to use SSH anyway and want to be selective as to what you stop, but minimise your rebuild time you can check to see what's using lots of resources using

# top

Don't stop services such as readynasd or apache, but any of your apps or file transfer protocols you don't need running you could stop if you want then turn on again afterwards.