Having at some point maintained a soft fork / patch-set for Nextcloud.. yes, there is so much performance left on the table. With a few basic patches the file manager, for example, sped up by magnitudes in terms of render speed.

The issue remains that the core itself feels like layers upon layers of encrusted code that instead of being fixed have just had another layer added ... "something fundamental wrong? Just add Redis as a dependency. Does it help? Unsure. Let's add something else. Don't like having the config in a db? Let's move some of it to ini files (or vice versa)..etc..etc." it feels like that's the cycle and it ain't pretty and I don't trust the result at all. Eventually abandoned the project.

Edit: at some point I reckon some part of the ecosystem recognised some of these issues and hence Owncloud remade a large part of the fundamentals in Golang. It remains unknown to me whether this sorted things or not. All of these projects feel like they suffer badly from "overbuild".

Edit-edit: another layer to add to the mix is that the "overbuild" situation is probably largely what allows the hosting economy around these open source solutions to thrive since Nextcloud and co. are so over-engineered and badly documented that they -require- a dedicated sys-admin team to run well.