Some games are just bad implemented. For example, i made a simple platformer game that lagged. Here's why:
Your graphic processor, processes images. that means to shade pixels if needed, or bighten them up. I wanted to implement a lighting system, so i had to brighten/dim the pixels. That meant, that i processed the image once to cast the lighting in the room. After that, for each object in the room, i calculated the shadow trajectory. Now for some math calculations.
The game ran on 60 fps. Each frame, i processed the image once for the light, and then once for each object in the room. If i had 10 objects in the room, that meant 10 + 1 image processing.
60 fps times 11 = 660 image processings per second.
The graphic processor alows you max. 64 arithmetic instructions per pixel. I used about 60.
An image has 1024 x 800 = 819200 pixels.
819200 pixels times 60 arithmetic instructions for each = 49 152 000 arithmetic calculations per image.
Now remember i had 660 image procecessed per second, that means i used 32 440 320 000 arithmetic instructions per second. 32.5 billion arithmetic calculations per second is very much even for your awesome pc!. And remember that those were purely for the lighting system! . Your pc has to calculate also the rest of the game...
Also, when it comes to mmo, there's a whole lot of data to download, about many many players. Maybe your bandwidth is not big enough