Same thing here (v20a 4.4.2 root)
Tested Sonic 2 and Sonic CD. These games ran flawlessly two days ago, when I was still in 4.2.2...
FYI same problem with "Leo's Fortune" now, which also ran fine before the update
No such problem in "Type Rider" for instance (similar gameplay)
These are the only ones I tested so far
Any problem with other games ?
edit : When it happens, pressing Home button then going back to the game gives another 20 seconds of smooth gameplay, but then...
In another (french) forum, a guy reported having this issue with 4.4.2 stock, but not anymore after installing CM11
Sounds like LG software might be the problem, not KitKat