Nextcloud very slow and/or not responding at all after viewing some files

Nextcloud version (eg, 18.0.2): 19.0.6
Operating system and version (eg, Ubuntu 20.04): Ubuntu 18.04
Apache or nginx version (eg, Apache 2.4.25): nginx 1.14.0
PHP version (eg, 7.1): 7.4.13

The issue you are facing: The nextcloud instance has multiple folders, which al have .mp4 video files in them, after viewing 10-20 of those mp4s for 1-2 seconds each the nextcloud is first getting slow, then after 2-3 more mp4s its not responding at all (only circles spinning for minutes)

Is this the first time you’ve seen this error? (Y/N): Y

The output of your Nextcloud log in Admin > Logging:

No log at all there, loglevel was set to 0, I've changed that to 4, nothing changes

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

<?php
$CONFIG = array (
  'instanceid' => '***',
  'passwordsalt' => '***',
  'secret' => '***',
  'default_language' => 'de_DE',
  'default_locale' => 'de_DE',
  'trusted_domains' => 
  array (
    0 => '***',
  ),
  'datadirectory' => '/var/www/nextcloud/data',
  'overwrite.cli.url' => '***',
  'dbtype' => 'mysql',
  'version' => '19.0.6.2',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'mysql.utf8mb4' => true,
  'dbuser' => '***',
  'dbpassword' => '***',
  'installed' => true,
  'memcache.local' => '\\OC\\Memcache\\APCu',
  'ldapIgnoreNamingRules' => false,
  'ldapProviderFactory' => '\\OCA\\User_LDAP\\LDAPProviderFactory',
  'maintenance' => false,
  'theme' => '',
  'logfile' => '/var/log/nextcloud.log',
  'log_type' => 'file',
  'loglevel' => 4,
  'updater.release.channel' => 'stable',
  'mail_from_address' => 'nextcloud',
  'mail_smtpmode' => 'smtp',
  'mail_sendmailmode' => 'smtp',
  'mail_domain' => '***',
  'mail_smtphost' => '***',
  'mail_smtpport' => '587',
  'mail_smtpauthtype' => 'LOGIN',
  'mail_smtpauth' => 1,
  'mail_smtpname' => '***',
  'mail_smtppassword' => '***',
  'mail_smtpsecure' => 'tls',
  'updater.secret' => '***',
);

The output of your Apache/nginx/system log in /var/log/____:

no errors in /var/log/nginx/error.log
(it's the correct file)

I can’t find any errors whatsoever

Just for video files or as well for others? Do you use previews of the file symbols (preview-generator)? The second access is faster?