As soon as the stylesheets vary depending on user permissions or settings the CSS and/or JS has to be recompiled very often under circumstances.

Example:

if (Piwik::hasUserSuperUserAccess()) {
 $stylesheets[] = "libs/jquery/stylesheets/jquery.jscrollpane.css";
}

if ($setting->isEnabled()) {
 $stylesheets[] = "libs/jquery/stylesheets/jquery.jscrollpane.css";
}

Imagine there are multiple users active and all of them have different permissions and/or settings and/or ... For each request we might have to recompile the CSS / JS as the registered stylesheets / jsFiles are different for each user.

This was for instance the case in #5212 but solved with a "workaround". Nonetheless we (or plugin developers) might face this problem again or have already resulting in slow speed from time to time.

As a plugin developer I would not be aware of such an impact when adding files depending on different permissions etc. Plugin developers would probably also not be able to reproduce this behavior as it only happens if there are multiple active users on a Piwik instance.

Maybe we can keep different versions of the merged files depending on the CacheBuster value. For example instead of only one asset_manager_global_css.css we have asset_manager_global_css_$CACHE_BUSTER_HASH.css. Not sure about side effects etc. There might be better solutions.