Adding hd slow

No.
What firmware are you running?
Do you have a backup?
Are all the disks' smart data ok?
Do you have an encrypted volume?
I would let it run to completion, then power down your nas, remove the disks, label them according to slot number, then test them all using vendor tools extended tests.

Firmware current. Smart data ok. No encryption. New drive added Friday 4pm build finished Sunday 430am. Smart error count on all 3 drive is 0. Just find it really slow. Didnt know if this was the norm or not.

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It is longer than mine takes (about 12 hours for a resync with raid 6, 5 drives)
Were you using the nas for anything during that 36 hour period? That would certainly slow it down.

No it was idle for most of it. Maybe a couple hours at night for plex.

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Everything seems to be running fine. Just found it really slow when I added the third drive to the system. I should probably expect it again when I get to drives 4, 5, and 6? The first two were pretty full it it makes a differance. I had about 500 or so gig free out of the 4tb or 3.6 useable. I have all 4tb wd reds.

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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

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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.