That is not something I've seen. I usually leave my computer to spectate in the background while I do other things, so I play game after game after game. I don't get any slowdown.
Is it summer over there? There is a good chance your computer is overheating.
Yeah the CPU core thing is just a lack of developer time to do everything that would be required. Our technically skilled developers are basically treading water just trying to keep everything stable (Which is very frustrating). We need more help if we want to expand the technical capacity of the game.