This is not really good. Normally you should get a login within less than 2s. However, it is strongly recommended to use caching and also adjust the mysql cache settings. For this many users, you can’t use a hosting system where you can’t control all these parameters. To get a correct feeling of Nextcloud, I’d suggest that you set up a virtual machine at your place (use a normal Linux distribution). Perhaps try a few settings and test with a couple of users how it looks and feels (your home network connection should be enough for this purpose). You can already measure the network traffic and monitor the performance to estimate what you need for a full roll-out.
For a few tests virtual servers or something at your home is a nice start, for the final solution, it’s probably better to use a dedicated server with a good network connection. First servers start at 30€/month.
Please note that the collabora docker image has some limitations:
There is an alternative way for install which should not have any limitations (not sure how reliable it works):