CSS Error with cssresourceloader & Slow Performance

Since last (?) version my nextcloud is really slow.

Also, in the nextcloud.log i get this error:
{“reqId”:“XrKzF0sVya2RX2CSNQAfTwAAEgc”,“level”:2,“time”:“2020-05-06T12:53:26+00:00”,“remoteAddr”:“78”,“user”:“admin”,“app”:“core”,“method”:“GET”,“url”:"/index.php/settings/user",“message”:“Failed to compile and/or save /var/www/nextcloud/core/css/css-variables.scss”,“userAg
{“reqId”:“XrKzF0sVya2RX2CSNQAfTwAAEgc”,“level”:3,“time”:“2020-05-06T12:53:26+00:00”,“remoteAddr”:“78”,“user”:“admin”,“app”:“cssresourceloader”,“method”:“GET”,“url”:”/index.php/settings/user",“message”:“Could not find resource css/css-variables.css to load”,“userAgent”:"Mozi

  1. Is it possible that the slow performance has something to do with the error?
  2. How do I fix the error?
  3. How can I analyze why nextcloud is slow?

Nextcloud version: 18.0.4.2
Operating system and version: nextcloudpi 4.14.150-odroidxu4 #2 SMP PREEMPT 9 armv7l GNU/Linux
Apache or nginx version: Apache/2.4.38 (Debian)
PHP version: PHP 7.3.14-1~deb10u1

The output of your config.php:

<?php $CONFIG = array ( 'passwordsalt' => '', 'secret' => '', 'trusted_domains' => array ( 0 => 'localhost', 5 => 'nextcloudpi.local', 1 => '1', 2 => 'n', 4 => '', 7 => 'nextcloudpi', 8 => 'nextcloudpi.lan', ), 'datadirectory' => '/media/k/ncdata', 'overwrite.cli.url' => '', 'dbtype' => 'mysql', 'tempdirectory' => '/tmp', 'version' => '18.0.4.2', 'dbname' => 'nextcloud', 'dbhost' => 'localhost', 'dbport' => '', 'dbtableprefix' => 'oc_', 'mysql.utf8mb4' => true, 'dbuser' => 'ncadmin', 'dbpassword' => '', 'installed' => true, 'instanceid' => '', 'memcache.local' => '\\OC\\Memcache\\Redis', 'memcache.locking' => '\\OC\\Memcache\\Redis', 'redis' => array ( 'host' => '/var/run/redis/redis.sock', 'port' => 0, 'timeout' => 0.0, 'password' => '=', ), 'mail_smtpmode' => 'sendmail', 'mail_smtpauthtype' => 'LOGIN', 'mail_from_address' => 'admin', 'mail_domain' => 'ownyourbits.com', 'overwriteprotocol' => 'https', 'maintenance' => false, 'logfile' => '/var/log/nextcloud.log', 'theme' => '', 'loglevel' => '2', 'jpeg_quality' => '60', 'log_type' => 'file', 'has_rebuilt_cache' => true, );

I solved it by applying this solution: https://help.nextcloud.com/t/file-is-locked-how-to-unlock/1883/46