You probably don't need your top vent fully open at all times - what temperature are you trying to get your BBQ to? Also, are you using anything like the minion or snake method for coals? What are you trying to cook on there?
I'm trying to cook standard BBQ food like burgers, kebabs, and sausages so lay out the coals flat across the grill rather than using indirect methods for long term cooking. I wanted to perfect the basics before moving on to indirect cooking of things like roast chickens.
Bit of an itchy head moment when the coals lose the bulk of the heat down to less than 100c within 20 mins of pouring them into the BBQ.