I enabled htaccess and now nextcloud is very slow although working fine.
sudo -u www-data php occ maintenance:update:htaccess
Have you checked in config.php the parameters:
'overwrite.cli.url' => 'https://cloud.domain.com/',
'htaccess.RewriteBase' => '/',yes , but Rewrite base to 'htaccess.RewriteBase' => '/nextcloud',
because my 'overwrite.cli.url' => 'https://cloud.domain.com/nextcloud',
That was exactly the same problem I suffer 
Could you try setting up Nextcloud in the root of webserver?
If you see a topic I created few days ago… In the log you could see some GET entries has /nextcloud twice. I thinks is a backend bug
well at the root I have different web app loading.
What did change in the .htaccess?
When you open a thread, there is normally a template, so you don’t forget to mention the version you are using, the logs etc.