I installed it on mine just to see what it does. It actually seemed to slow down the loading of my Roundcube webmail pages. At least initially, anyway. I will test it out during the day and see if there is any noticeable difference.
As a side note, using firebug and the pagespeed extension for firefox to analyze my Roundcube webmail pages, it suggested using compression like mod gzip would improve loading some elements by 70% or so.
I will now investigate whether mod gzip will have any impact on load.