Me and another newbie were doing 2v1 against someone who isn't as bad. The game resulted in a near stalemate with him trapped in his base and us slowly bringing in gold to upgrade gear to kill him with.
After what must have been a couple hours the game started to lag *really* badly. Everyone had a reasonable ping (under 100-200) and Sim Speeds were between 7 and 10.
For me, my frame rate was fine (animations were smooth, etc.) but the game would play smoothly for about 1 second and then it would pause for about 5+ seconds. This repeated for what must have been 45 more minutes (which equated to only a few minutes of game time). What is odd is that it got progressively worse the longer the game went on even though nothing was changing (number of creeps on screen were the same as the citadel was reflecting damage back at them, killing them faster than they could kill it).
The only reason I stuck around was because I didn't want to take a loss. Perhaps our opponent, who also didn't want a loss, alt-tabbed out which is what caused the problem? If so the Sim Speed should accurately list him at 0/1. Whatever the cause was the game was definately not enjoyable for the majority of it. 
