Even with the "close client during game" ticked, the game is still highly unoptimised compared to its predecessor.
I used to be able to run the game with medium to high setting (yes, on the new summoner's rift, until right before the old client is retired) with stable 60FPS. I had high setting texture, low or medium shadow, I could turn on character-inking, I could even turn on vsync and faced no issues whatsoever. They really weren't kidding to say that the game is literally run-able with 2 toasters connected in series (I kid, but you get the point).
And now, to play this game, I have to use low settings, turn off many things, the shadow is off altogether on my comp now and texture setting goes to low. And the FPS is 60 at base, but it drops to 40s as soon as there's any action on the screen. In teamfight it's even worse, it can drop to 20s. That's real bad when compared to the old edition where it was flat out better performance wise.
Let's not even mention the post-game loading if you have the "close client during game" ticked. It takes about a minute to load into the post game chat. That's really, really bad. It's even worse where at one point they actually fixed it to around 20 seconds in one of the recent patches, but nope, it's back to one minute or 2 now. Sure, re-opening the client takes time, but let's all be real, how much stuff do you think the League clients need to do during launch, and even if there's a shit ton of stuff can't you just load the part where it matters and then load the rest of it behind the scene or after the scoreboard?
Please optimise the new client. This is really annoying.