KVM/libvirt VM with 4GB RAM and 4vCores shared with a dozen other services, storage is not the fastest (qcow2-backed disks on a ext4 partition inside a LUKS volume on a 5400RPM hard drive... I might move it so a SSD sometime soon) so features highly dependent on disk I/O (thumbnailing) are sometimes sluggish. There is an occasional slowdown, I suppose caused by APCu caches periodically being dropped, but once a page is loaded and the cache is warmed up, it becomes fast again.
Standard apache + php-fpm + postgresql setup as described in the Nextcloud official documentation, automated through this ansible role