I'm pretty sure the diff change would have no effect on your miners speed. Did you try a reboot?
Dunno. I rebooted a couple of them just the other day when Kano restarted something because miners were dropping off and on. I thought I had something going on here so I did some rebooting of my own before I saw his post. Heck one of them I even reset to factory defaults and reconfigured just to make sure.
I don't know what your issues may be. But if the diff changes were to slow down miners, my S9's would all be hashing below 5TH/s with all the diff changes they have seen. LOL
Good luck with it!