Maybe it is the graphics: Many times the problem lies in the graphics which influences the whole thing to lag.It is like a good chain that affects the whole. If the dolphin is lagging, so will the game. If your computer has less space then it could cause the game as well as the dolphin to lag. Maybe it is your computer: Many times as it happens the game that you are playing on your computer using the dolphin might require more space.
If the configuration setting is not set right then it will not work properly then it might cause the slowing down of the dolphin.