How do I change the game speed during a match in Starcraft 2 (SC2)?

Solution 1:

This can only be done in practice league. To do it, hit f10 --> options --> Gameplay, then slide the "game speed" slider to the desired speed.

You may also get a message saying "XXX is slowing down the game..." in a dialog box, which has to be closed by hand. This can happen even in league games - it is simply (a very poorly thought-out message) trying to explain why any perceived lag is occuring.

Solution 2:

As you mentioned, you were in the practice league still. There is no way to change the game speed of a ranked ladder match or placement match.

I'll try to look into how you can do it while in practice league, but it will be made a tad more difficult by me not being in it.