It is also compounded by latency issues and such over the web. The server has to relay the positioning and such for all moving models on your screen--be that players, NPC's, Minions, Chocobo's, Monsters, etc--everything that you and/or other players might interact with requires a response from the server since it is not managed client side and then synched.

It's a combination of a LOT of things adding up to a slow down in heavily congested areas, and not all of it is bound to your platform's hardware.