According to cPanel support, if your Roundcube is slow, you should read this:
https://support.cpanel.net/hc/en-us/articles/360049982694-Webmail-Login-stuck-on-Logging-you-in
And do this:
cat /usr/local/cpanel/logs/php-fpm/error.log
Look for:
WARNING: [pool user_username] server reached max_children setting (25), consider raising it
Then do this:
# cat /var/cpanel/php-fpm.d/username.conf | grep pm.max_children pm.max_children = 25
Update that value.
Then do this:
/scripts/restartsrv_cpanel_php_fpm
Surprise, surprise, it doesn’t work. The clue is in the helpful count at the bottom of the article.