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Ref: https://lowendbox.com/blog/how-to-speed-up-your-software-raid-resyncs-thanks-lowendtalk/

First, turn off all other activity on the box so RAID resync is the only thing going on.

Next, set /proc/sys/dev/raid/speed_limit_max and /proc/sys/dev/raid/speed_limit_min to higher values.  Out of the box, speed_limit_max was set to 600000 which wasn’t bad.  Setting it to 700000 didn’t speed things up in my case, but if you have a very low value (or no value), it might help.

Modify these commands to suit your needs:

echo 600000 > /proc/sys/dev/raid/speed_limit_max
echo 100000 > /proc/sys/dev/raid/speed_limit_min

Finally, modify /sys/block/md2/md/stripe_cache_size so the system can work in bigger chunks.  A lot of systems default to 512.  Synology defaults to 1024.  I bumped it up to 4096.  Note that this command is RAID-device specific (in this case, my md2 RAID device) – so don’t copy/paste this command blindly.

echo 4096 > /sys/block/md2/md/stripe_cache_size

I wouldn’t leave it set at that forever but for resyncs, it seemed to help a bit.

Cumulatively, these settings improved performance by about 15%.   I’d guess the stripe_cache_size was the main contributor to the improvement.  Since the system is idle except for the resync, there shouldn’t be any other work on the disk fighting for I/O where speed_limit_min would come into the picture.

Two days later, my RAID resync is still running but these tricks shaved about 8 hours off of it.

Thanks to @luckypenguin for the tips!