Were you running any other applications in the background? And again, which are your system specs?
The game is usually more CPU intensive than GPU intensive, but if you have low-end rigs for any of them, the game will probably be slow.
However, the lag can be caused by external applications too, like the ones mentioned in the second post here: Crashing on start? Read this! UPDATED October 18, 2016.
Were you using any mods?