it's the air lock phenomenon in pipeworks. i saw a vid explaining it on yourtube, with help of transparent piping to illustrate
same deal in radiator end tanks and high points in loops. it creates big flow restrictions.
Last time i refilled the loop, it almost halves the flow until i start tilting the case to remove the air. it's big

When I converted my system, a few weeks ago, to hardtubes I was after filling and first venting at 65 l/h, after I turned the airplex radical then again individually it went up to 107 l/h, I have the feeling that tube radiators are more vulnerable than mesh radiators, such as my 3 NexXxoS XT45.When I converted my system, a few weeks ago, to hardtubes I was after filling and first venting at 65 l/h, after I turned the airplex radical then again individually it went up to 107 l/h, I have the feeling that tube radiators are more vulnerable than mesh radiators, such as my 3 NexXxoS XT45.

Probably the net radiators do have the higher flow resistance.