QUOTE(Cheesenium @ Nov 6 2009, 03:38 PM)
It's always like that,because there is a need of having all the calculations synchronised.
If one computer lag,everyone lags.
Erm no, its mainly because dedicated servers don't actually have to do all the other calculations that make a game, a game.
It only needs to have an abstraction of the "map" and "dots" representing the information about each player and perhaps some extra calculations for dynamic effects on the map (Like the train in TF2, the round timer, etc.)
It does not do any 3D rendering which is where the bulk of a game's processing power is required.
As a result, a simple server can manage multiple hosts at once without having performance issues (Long time ago, during LAN parties, we'd use ridiculously old PCs to act as dedicated servers and they performed fine).
When you're using Client-server (P2P is actually much different, AFAIK, only Demigod uses this method), the host PC has to manage all that information ON TOP of handling the actual game all at once. This means, the "servers" overall performance is severely reduced as a result.
The whole "calculations synchronised" part is true for all multiplayer games and whether or not its using dedicated servers or Client-server hosts, it will be the same, just that Client-server hosts need to run the game at the same time which reduces its overall ability to function as a server.