Multiplayer modes like that are more demanding in Dolphin. Dolphin literally has more things to emulate when you start adding more players in those games. You don't have a particularly strong CPU, so you see slowdowns as a result.
4.0 is slower than the latest development revisions, and in fact it's not supported anymore (4.0.2 and newer is, but please don't try 4.0.2, it's just as slow as 4.0) Go here and download the first version you see: http://dolphin-emu.org/download
Also, google "dolphin emu laptop performance guide" and try the tips listed in that thread.
4.0 is slower than the latest development revisions, and in fact it's not supported anymore (4.0.2 and newer is, but please don't try 4.0.2, it's just as slow as 4.0) Go here and download the first version you see: http://dolphin-emu.org/download
Also, google "dolphin emu laptop performance guide" and try the tips listed in that thread.