If you ever have to move a Webtrees install to a new server, make sure your data/cache folder and files within are given the right user:group ownership. I was seeing 6-7 seconds per page load with all modules off, because thumbnails etc. weren’t able to be cached and were being rebuilt every page load. Yikes!
For example, on YuNoHost, I ran this command: chown -R webtrees:www-data cache, which aligns with the user:group as installed.