Recently, I installed NCP on a Raspberry Pi 4 with an SSD drive attached. Loading pages of the webinterface of nextcloud is incredibly slow and sometimes, it lasts several minutes to load a page.
While waiting, I observe the processes with top and some apache2 processes are doing something with less than 1% cpu. Then, when the page finally loads, php-fpm is kicking in.
Is your ssd powerd via usb? If so, try connecting it via a self powered usb hub.
Also, link to, output of ncp-report on pastebin or similar, could be useful.