Server-too-slow error occurring after watching loads of movies

I accidentally clicked 2 or 3 times on the “play” button on amazon fire tv stick app - now the whole plex installation is broken. On Amazon Fire I get the error “…connection to plex media server has been lose” when starting a movie, and on web-interface I get the server-too-slow error. As the server did not get slower through clicking the play button multiple times and restarting the server ans stick does not resolve the problem there seems to be a configuration problem. “Optimize” has a speed of 1.9 to 2.2x, which also states that this message is not true. How to resolve this issue?

(as this is a dual core Synology NAS server it actually could be true while some other app uses all the cpu power…but again, no change to before I clicked too often on play.)