My configuration is slightly different (debian, mariadb, nginx) but otherwise I have similar problem as you (Nextcloud web-interface too slow). I tried everything: more and faster cpu, more ram, fast ssd, added all possible caching, tweaked php/nginx config for more servers/workers, deactivated many modules, but nothing helped.
I can not even find what actually is reason for NC web-interface being so slow. My vps is basically sitting idle, and yet NC is slow. Very slow. Any other website I tested on the same vps was blazingly fast, but not NC. I’m starting to think this is not bug, but “feature” of NC web-interface…