I'd argue that the primary function of Nextcloud is to serve files. Of course the other services lack other stuff, which is why I'm still using Nextcloud. But I still wish its performance was similar to pure file servers.
PostgreSQL is definitely a boost to performance, especially if you offload the DB to a dedicated server (depending on load, can even be a cluster)
Nevertheless, it probably has much to do with how it's deployed and how many proxies are in front of it, and/or VPN. If you have large numbers of containers and small CPU/low memory hardware, and either running everything on one machine or have some other limitations, it'll be slow.
Admittedly, I'm not very familiar with the codebase, but I feel Apache isn't improving the speed either. Not exactly sure how PHP is nowadays with concurrency and async, but generally a microservice type architecture is nice because you can add more workers/instances wherever a bottleneck emerges.
Dropbox is A LOT faster than NC ever was. But if you want to talk about speeds and reliability then use Synching. Add FileBrowser if you want to have a WebUI on a central “server” to access all your files and you’ll be 100x better than the garbage that NC offers.
Run it on a “normal” server and everything is smooth.
Sure until you try with a high end 12 core CPU on NVMe storage all kinds of caching, redis etc. and you find you it doesn't perform particularly better.