Nothing reported in the admin screen listing out config tips and missing indexes?

To be totally fair, in my long history with it, I've been complaining more about the bad performance of nextcloud than the opposite (I still do), but things improved quite significantly after I moved (a long time ago) to postgresql+pgbouncer, which made it acceptable. Tuning did the rest.

There can be several reasons why your experience with nextcloud is not optimal. For instance, if you have a slow mechanical drive and a weak 2-cores CPU like I do, it's enough to hit a large folder with many pictures for the first time to have it grind to a halt: the server will become IO and CPU starved while php-fpm fork bombs (on a default config) way too many imagemagick processes to render miniatures of the photos, which, when those are large, will make sure to eat the little amount of RAM you had left, further pushing you into swap and memory-compression territory, making things even worse IO & CPU-wise. This is easily mitigated by rendering the miniatures asynchronously via a cronjob and making sure that there is a reasonable limit to php-fpm's parallelism to keep your app, navigation and overall system responsive.

Honestly, this is basic sysadmin/tuning stuff, that only you (with the knowledge of your hardware) can do right (and should do, the reward is immense). And it might very well be that nextcloud is (much) more demanding than other apps due to its inherently bad design & stack. It does the job, though, and isn't afraid of taking hundreds of thousands of files.