Slow network speeds to/from NAS.

I used to get transfer speeds to/from my NAS at ~100 MB/s. However, now, I get those speeds for about 10 seconds, then it instantly drops to about 30 MB/s. Unfortunately, trying to troubleshoot, I recently took a (seemlingly minor) firmware upgrade for my router (Netgear Nighthawk R7000) as well as reinitialized the NAS (that's another story).

Does anyone have advice on how I can troubleshoot my sudden change in file transfer speeds?

Update: I found a way to roll back the router firmware, and I am still having the slower speed. When I am transferring files (via FTP), the speed is still about 1/3 of what it was, and the server almost seems to be "choking" (less responsive doing other things via the web interface). Also, the fans on the server have been running a lot louder/faster lately.

Update 2: It turns out these lower speeds are related to enabling Cache accelleration (2 m.2 SSD drives set up as read-write RAID 1). They were still synchronizing, which I am hoping was the cause. After disabling the cache acceleration, my speeds significantly improved. I will re-enable after the RAID synchronization is complete, hopefully giving me better speeds again.