Arne Berglund wrote:
The database you use will also have an effect, as will the response and performance of the IMAP server. But in general, Dave is correct - Roundcube itself is not a process that you can monitor or scale, you will need to scale the web server and database to the number of users.
Not forgetting the IMAP server that does the real work. I expect that'll be where the bottlenecks are to be found, if any.