Basically the web interface is not usable and takes a long time to process each click. What has the community done to improve performance? I am using MySQL and Php-fpm 7
What is long? What have you done to improve the performance, caching in general helps a lot on the interface speed:
https://docs.nextcloud.com/server/11/admin_manual/configuration_server/server_tuning.html?highlight=performance
With the limited resources on the RPi, you must carefully share your resources. Monitor your server a bit and check out which processes take a lot of CPU/RAM and try to optimize it. On the database you can do quite a lot with the size of caches. mysqltuner and mysql-primer can help you to identify the critical settings (also too large values which waste your precious resources).