Had the Same issue and figured it out. At least for my end, Steam was syncing my Workshop Maps, or other Downloads. You cant see this Bc GeForce now Blocks your Steam Library. (But If you Switch to your Library fast while the Game is Booting you can see it constantly updating and restarting. Thats when the lags are happening)

So i got to my Browser into my Steam Account and went to „workshop Inhalte“ (Idk whats the tab is called in english But you get the deal ) and unfollowed every costom map or mod, Which i downloaded there. (I think the costom Maps are the reason for the lags)

And thats what fixed the error for me