Obviously, everyone will recommend Nextcloud. And yes, it works. But it is heavy and can feel slow.
I then moved to OwnCloud, the original project behind Nextcloud, which I found significantly faster. After that, I switched to OpenCloud, which is closely related to OwnCloud. It is less intuitive to set up, but it is extremely lightweight and very fast. That is the one I recommend.
Depending on your use case, there are plenty of other options as well, such as Seafile, Filerun, Filestash, Copyparty, and others.
Fair point, stability and long term maintenance absolutely matter.
I was mainly highlighting performance because that is often the main friction people, including me, report with Nextcloud for simple setups.
OpenCloud is not a random new project either, it comes out of the ownCloud ecosystem, so there is some history and structure behind it.
If you have specific alternatives you consider stronger in terms of governance or long term stability, I would be interested to hear them. It would help broaden the list of solid open source alternatives.