I personally haven't had any slowness due to size. I have had slowness due to the algorithms I tried to put into place.
Perhaps someone else has experience with that (or perhaps Alex could answer what the memory requirements would be roughly for objects, attributes, etc). I have had a concern about the Undo information saved per turn, as that could grow without bounds, impacting larger games.
I also find that the web player is slower than the desktop one, both in terms of the turnaround needed for the screen to update as well as the script execution, possibly because the Quest servers are slower or more loaded than my laptop.