My game keeps lagging hard.

Hi,
This is the second league that I really play till endgame. And the last time I played it all went fine. This time, after about a week of playtime I started to get some lag spiked. It was nothing too serious and I just rolled with it. But it kept getting worse and worse.
Today I logged in and it became impossible to play the game suddenly. The game will freeze for multiple seconds. I feel kind of frustrated.
I dont have these issues with any other game that I play, including diablo which is in the same genre.

And you would think the lags would happen when a lot of mobs are on screen. but thats not the case. It also happens when im alone in my HO.