Weird performance issue with Assassin's Creed on Laptop with GTX 1060

So I have bought a new laptop relatively recently: an Asus Rog GL502vm which has a GTX 1080, i5-7300hq, and 8GB of RAM. So I decided to put it through its passes, and installed Assassin's Creed Origins.

The game automatically set graphics details to very high, but when I ran it the frame rate was abysmal: 15-20 fps. After fiddling a lot with it I realized that if I alt-tab out of the game after it loaded, and back in, the fps jumps to 45-60 and is actually playable.

As far as I can tell the laptop only has the GTX 1060, no Intel Graphics, so I don't think it's using the wrong GPU until I alt-tab.

So what gives? Why is this happening, and how can I make it work without having to alt-tab each time I open the game?