I have the same problem Plex movies are buffering every minute. Even when I transferring files to My Cloud Home, it’s pending from zero to 90 MB/s in every 5 second. Something with this update is charge the load to the CPU on My Cloud Home?
Download the debug_logs.tar which will show the state of the HDD in smartctl log, whether there are errors in the HDD and separately the Plex logs which will show other network and transcoding errors.
The MCH is a light duty personal storage that is less likely to be durable much longer than the 2 year warranty when used as a heavy duty server in business or media. It is the property of the SMR HDD found in most MCHs.