It's very odd, but i've got extreme lags during playing game. I have 60 fps, flying, walking is perfect. the problem appears when i start crafting or building. Blocks appear with 1-3 sec. lag. The same thing when i'm using inventory. Lags can be even 10 sec. My processor usage is normal (not more than 50%).
I use direct 8 and no fancy graphic features...
sorry about my english...