LoL is running quite badly, given my specs

So, I have a GTX 1060 6GB, a Core i5 (8th Gen) with 3.9GHz and 8GB of RAM. The laptop's screen is 120GHz and I am running Windows 10. I should be able to easily keep the game at 120 FPS, without it never getting lower, I should actually get way more than 120 FPS, but for some reason, the FPS are always going up and down between 50-115... I have to say that Alt-Tabbing in and out of the game takes a considerable amount of time, with me getting a black screen with only LoL's cursor for about 10 seconds when Tabbing in the game again. Before purchasing this laptop I had another one, without a dedicated graphics card, 8gb of ram and 3.2 GHz and it could run the game smoothly at about 140 stable FPS, no problems with alt tabbing, but it ran Windows 8.1. Could the OS be the problem?
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