I have Plex, Sonarr, Radarr, Readarr, Calibre,... all running on the same server.
I can stream plex, import movie files, ect all jobs are running (normally) fast.
So I don't think it is a network or CPU issue.
http://localIP:8081/ajax/books/books
Has a TTFB of 2.1min and Content Download of 869.27ms
As a comparison Radarr :
TTFB 20.53ms and Content Download at 1.3ms