Hi
I like how Nethserver gives you the options to easily set up a Nextcloud instance.
I would recommend to switch from Apache webserver to Nginx for Nextcloud.
Jus by switching to Nginx made a very big difference on my custom Nextcloud 12 VM (Ubuntu 16.04.3 LTS)
I am using HTTPS only. The speed improvement might come from using HTTP/2.0 also.
After this i added the PHP OPcache, APCu memcache + disabled bruteforce check (yes i know that it weakens security) made my Nextcloud run a lot faster and be really usable.
This might not look like an easy task but it is worth it.
With this message i just wanted to let you know of my experience with Nextcloud.