Why is there a direct performance difference when playing in custom rooms compared to normal games no matter the map or amount of players or the ping of the players?

A full room on normal gets way more fluid gameplay regardless of the custom room setup comparison (etc. 1vs1 or 2vs2 on the same server/map/mode or even smaller map).

Edit: Valofe changed its focus towards custom games but this problem have been apparent already since Valofe took over the service yet nothing seems to have changed regarding the performance difference.
So my other question is why Valofe switched over towards custom rooms when they should already know that this problem exists and that it would only get worse now.