Here's the game: https://www.roblox.com/games/439514611/SKATEBOARD-SIMULATOR
If you view the performance stats you can see the the physics tab is completely red (or orange) and it seems to be quite lagging...
EDIT: Okay it's not always red, somewhere in the level 7 it starts to rise up
Is it bad? And is there anyways to prevent this, because I don't want to quit making my game just because of the lag...
I played your game just now and I think it might be cause of all the spinning coins. Make them into billboard GUIs instead, the spinning might be causing the physics lag.