What kind of NASes do you have? I would check the SMART stats for every disk in every NAS. I’ve seen QNAPs that refuse to evict disks with 50k bad sectors and the whole unit grinds to a halt. Checking SMART is also going to be super fast compared to adding / removing NASes and trying to grok the effect that has from the Chia logs.
Logging into the GUI to check SMART on each NAS will also help IMO. If you have identical NASes you might be able to notice a performance difference just from logging in which would help narrow it down too.
I’m interested to hear what it is when you figure it out. I’d put my bets on having a bad disk in the first NAS you filled up.
Good luck!