what is happening

Okay. When i power on my pc, in the beginning it's slow. Okay i get that. When i start playing with it, or just browse for 30 minutes it's fast. Very fast. After that i close everything and leave it on the desktop and go out. After 1 hour after i come back i try to do something on my pc and its...

I'm just looking for micro atx cases that will support bulky coolers such as the dark rock pro 4 or the noctua nh-15, multiple reccomendations would be nice as I like variety. I really need a case that has excellent airflow, i dont need a case for RGB or tempered glass.