Plex Media Server beta 1.17.0.1709 released earlier today has changes that should reduce the number of times the EPG is refreshed when XMLTV is used.
See Release Notice here Plex Media Server - #286 by emilybersin
- (DVR) Enhanced Plex Media Server’s performance by skipping reads of previously processed XMLTV guide data. (#9846)
I believe it now checks the date and time of the file and would only process it if there have been changes
