Nextcloud time to first byte (TTFB) too long - enable caching

Apparently, there is a redis server running on my stackspin machine - but where is it integrated? Nextcloud and Zulip?

I am having issues with long TTFBs (a few seconds) in Nextcloud, which is driving users mad, and was wondering where I could tweak it to improve performance and especially cache unchanged calendars.

The documentation has no mention of redis and elasticsearch - though both run for Zulip afaik now.
Then the latter could also be used to enable Nextcloud full-text-search, which I will happily contribute when I know the technical underpinnings.