nextcloud just feels abandoned, even if it isn't of course.
maybe paying customers are getting a different/updated/tuned version of it. maybe not. but the only thing that keeps me using it is there isn't any real selfhosted alternatives.
why is it slow? if you just blink or take a breath, it touches the database. years ago i've tried to optimise it a bit and noticed that there are horrible amount of DB transactions there without any apparent reason.
also, the android client is so broken...
I'm not sure why you feel like it is abandoned. There is a steady release cadence and the changelog[0] clearly shows that much is being worked on.
[0]: https://nextcloud.com/changelog/#latest32