It could be caused by server lag or game optimisation if its not happening in any other games.
"Low FPS and lag sometimes show up at the same time, complicating things. For example, if you’re playing a massively multiplayer online RPG (MMORPG) and your character is standing in a major city with hundreds of other characters running around on your screen, you may experience both low FPS and lag. The lag occurs because the server is struggling to keep up and slowing down with so much happening in one place at once, while the low FPS is a result of your computer struggling to draw all those characters on your screen at once.
In this case, low FPS and lag have the same cause – more people than the game can handle in one place at the same time – which may confuse some gamers into thinking they’re the same thing. But they’re not – the large number of players is causing both performance problems on your computer (low FPS) and network issues (lag)."