Well, the load speed depends on three things mainly, which can then be broken down into another few things.
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1. Server
A. Connection Speed (location is a factor here too)
B. # of clients connected
C. amount of data going in/out
D. The server computer's specs (CPU, RAM, etc).
2. Client
A. Connection Speed
B. amount of data comming in (data going out here isn't that much of a problem for the client vs. the server).
C. The client computer's specs
3. Game (the actual online game)
A. Size of program
B. Size of media that needs to be loaded (models, textures, etc).
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For connection speeds, it really depends on how you have data organized and transfered. And for game speeds it what the computer has to process.
What a lot of the big online games do is have multiple game servers, but have the permanent data be sent to one main server (like a login server). How the multiple server model works is that the client tries to connect to the closest server to them, if it fails, it goes for the next one in the line.
There are also games like warcraft 3's online play, they have you login to one server, then from there you can play games that are hosted by other clients (know as map hosts in WC3 terms). So all the first server does it manage the userdata (like if a user is logged in or not, or how many game they have won), and then have actual clients host the games. I creates a lot less server load.
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