What's wrong with Nextcloud, and why is it slow/clunky?
Yes it's a pile of shit. Nextcloud is garbage, very bad usability, more reasons and issues listed here: https://lemmy.world/comment/1571886 and https://lemmy.world/comment/346174
Is there a technical reason why it’s slow and clunky? Any problematic choices with how it was built?
Yes, like every single technical decision and line of code they've ever made. https://lemmy.world/comment/5490189
There's software that while good intended is simply garbage and NextCloud is a good example. They constantly market themselves as the self-hosted alternative to MS Office 365 / Google yet they never deliver.