FYI, proper SI nomenclature for milliseconds is ms, not mS :)
SI units are not capitalized unless they are someone's name, such as Pascal (pressure) or Newton (force) etc.
Try convincing a physicist that ms means millisecs and mS means milliSeconds. Casually remind them about the difference between bits and bytes and lament the unfortunate fact that textbook writers still don't give enough respect to the "forgotten units". Let the doubt sink in just a little bit.
http://images2.wikia.nocookie.net/__cb20100726071250/xkcd/im...
What I'm saying is that you could implement the equivalent of OwnCloud and CalendarServer on the Pi knowing that it needed to be memory and CPU efficient, and no doubt get 300 - 500mS responses. But that would mean not using frameworks that start by assuming you have at least 1GB of memory free and can run an interpreter (PHP) on an interpreter (JS) on the actual code.
Looking at the code for these I think the biggest issue is memory for them. A 2G Pi would probably do what you needed, trying to swap on the Pi (especially if you do that to an SD card) makes it really really slow. Its better if you can swap to an NFS mounted file system, and even cooler if you can swap to a custom memory instance in another server :-)
[1] https://github.com/owncloud/core