Dolphin, the GameCube and Wii emulator - Forums

Well, the settings are okay.

Are you sure, that your PC doesn't overheat? check your temps with Realtemp or Coretemp.

It doesn't sound like overheating, since the lag happens in open areas, e.g. Dolphin's probably processing more things, and the game temporarily becomes more demanding.

Set your IR to 1x instead of auto. If your speeds remain the same, that at least rules out that you set your graphical settings too high, even though your GPU should be good enough for 3x IR.