My guess would be Nextcloud put a lot of load on both CPU and disk (for thumbnail generation, file encryption, etc.) making Raspberry Pi which lack performance in both front not very suitable for the task.

I tried running Nextcloud on Raspberry Pi 4 (4GB) a while ago, and hitting performance issue on SD card very fast (initial DB import take very long). I tried to solve this by moving all Nextcloud data into iSCSI disk but RAM & CPU became a problem once I started adding more files. After a week or so, I tore everything down and went back to a VPS instance.

It might works better with Raspberry Pi cluster on Kubernetes, though. (This very Raspberry Pi is now happily running as a DVB streaming server.)

Try using a SSD connected via USB 3.0 on the RPi 4.

Thanks. I keep meaning to try it out but booting from an SSD instead. I was hoping anything intensive (e.g. thumbnail generation) would be a one-time thing so not noticeable in daily usage. Perhaps I will re-think, I have a ThinkCentre M75q I could use but that seems overkill.

I find you can get a much better experience with a cheap celeron NUC than a RPi because everything from single thread performance to IO speeds favor x86 right now.