i used to run Owncloud (php), which was ok for CalDAV and CardDAV, but the web interface was terribly slow. Now I run calendarserver for CalDAV/CardDAV (Python), which is fast for CardDAV (few data) and slow for CalDAV (a lot of data). I am also running Bittorrent Sync (binary blob, fast), and Time Machine backups (fast).

My conclusion: The Raspi is slow for serving with dynamic languages. Compiled stuff is fine.