Game lagging very badly after Building

Hey guys, so I have been working on a game for a few days. I pressed build and waited for a few hours while it worked. After it finished my game now lags very badly. Before I built it when I would hit play it would go right into the game and play with no lag at all. After building when I hit play it takes about 10-20 seconds to actually start and when it does it’s pretty much unplayable. Any help as to why this happened would be appreciated, thanks!