I experienced a very similar issue but after a successful update from NC19 to 20.
User interface became unusable, it took over 30sec before any page would load and disabling CollaboraOnline was an immediate remedy.
Running a code check on the reachdocuments app gives:
root@nextcloud:/var/www/nextcloud# sudo -u www-data php occ app:check-code richdocuments
Analysing /var/www/nextcloud/apps/richdocuments/appinfo/app.php
1 errors
line 51: OC_Helper - Static method of private class must not be called
Analysing /var/www/nextcloud/apps/richdocuments/lib/Controller/DocumentController.php
1 errors
line 565: OC_Helper - Static method of private class must not be called
Analysing /var/www/nextcloud/apps/richdocuments/lib/Controller/WopiController.php
1 errors
line 349: OC_User - Static method of private class must not be called
Analysing /var/www/nextcloud/apps/richdocuments/lib/Service/UserScopeService.php
2 errors
line 65: OC_Util - Static method of private class must not be called
line 66: OC_Util - Static method of private class must not be called
App is not compliant
and this is for the latest v3.7.10