i run nc 12.0.3 on debian 9.2, too (with apache, php7, mariadb). i confiugured opcache, but i “felt” (really did) the greatest performance increase after adjusting the (maria-)db-settings.
also, what really speeds up the gallery is preview-generation (which takes time, though, around 150-180minutes/night via cron).
i posted everything on these forums, but i just found that the tiltes are misleading:
https://help.nextcloud.com/t/slow-performance-generally-with-nc-12/21390/4
https://help.nextcloud.com/t/debian-9-1-admin-warnings-despite-opcache-running-clarification-on-settings/20881
GOOD LUCK!