That's impossible, you can't mine and play video game on the PC at the same time, it will surely reduce it's hashrate or even stop mining and hang your system, I don't believe this, I can go with youtube video and browsing but not gaming, even watching YouTube video while mining will slow down your hashrate
Try running mining on a video card and the game "Heroes 3". Hashrate will not drop. Of course, if you run games like Call of Duty: Modern Warfare,
you won't be able to mine and play at the same time. Friends miners play on farms in EVE Online. Perhaps FP91G was talking about such games.