VSR causes your game to render at a higher resolution, but then downsamples it to your native resolution. It would decrease FPS because you are essentially playing at a higher resolution. It all depends on the game. If you try to run it on Far Cry 4, you would notice a hit. A lighter game like League of Legends or Counter Strike might not take as much as a performance hit.
tl;dr it looks nicer but it will reduce framerate depending on the game.