If you weren't aware, Nextcloud provides non-AIO compose files in their docker repo: https://github.com/nextcloud/docker/blob/master/.examples/docker-compose/with-nginx-proxy/postgres/fpm/docker-compose.yml
NVMe drives aren't guaranteed to be fast. Based on those stats I'm guessing you have QLC and no DRAM.
If you weren't aware, Nextcloud provides non-AIO compose files in their docker repo: https://github.com/nextcloud/docker/blob/master/.examples/docker-compose/with-nginx-proxy/postgres/fpm/docker-compose.yml