From the server side, I don't think Nextcloud has a way built in, but you can do this with Apache or Nginx depending on which one you use although there is a learning curve there. You could also do this on the firewall depending on your firewall, again could be a learning curve there. If you happen to want Nginx bandwidth limiting settings (what I currently use) I'm happy to share those settings.
From the client side, the desktop client has an upload and download bandwidth setting in the Network area, but you would need to configure this for every client obviously. I'm not sure about the mobile clients.