New MacBook M3 Pro screen is lagging

Hello everyone. I’m so sorry for posting it. So I bought MacBook Pro M3 and it arrived just today I decided to get this one after all the other laptops I bought broke down after a few days (overheating etc). I chose Apple because of their reliability and I've run into a problem with a new machine, I'm desperate.

The problem is that when I turn on the laptop and log into my account the screen brightness seems to automatically change (during loading into my account process), the screen first becomes less bright for a second as if lagging and then becomes normal brightness again after the process is complete (takes some time). I turned off the auto brightness setting but it didn't help, when I reboot it's the same thing. Literally ready to cry. Is this a defect or not? The laptop is literally two hours old so I don't know if I need to send it back. Nothing helps and it hasn't happened to other laptops before.

Sorry for the rambling I'm very upset, because I just want it to be normal.

Thank you so much for your answers in advance.