FYI - I have a 3 M5 router mesh that has been in operation for a little over a year. I have an 8-port 1 gbs switch connected via ethernet cable to my main mesh router with 6 devices connected to the switch via CAT 5. I frequently have need to transfer very large (over 1 GB) files to my ethernet connected NAS. Recently, I noticed a substantial drop in the transfer  rate of the large files (down to abt. 355 kbps), small file transfer was still good. The problem only affected the CAT 5 connected devices, WiFi remained good. Commonly, for Windows 10 devices, this is a setting in the properties of the network adaptor(s) and can possibly be resolved by disabling the "large send offload" settings of the device, however, the NAS has no such settings and the problem occurred with multiple PCs attempting to transfer files. Eventually, I tracked the problem down to my main M5 router. Apparently, something went south in the nic for the device causing it to not handle larger files properly. I swapped the defective M5 with one of the mesh that had no ethernet connections. This solved my problem as the WiFi connectivity of the defective M5 remained good.

Maybe someone can be helped with this post.