@arminunruh Hi Armin.
I'm sorry to be bothering you with this - its just so mysterious.
I'm in Germany , Berlin - gigabit connection
Using Chrome
On Mac Chrome has no issues - even when displaying a mobile view
Phone is a modern mid-tier device -Pixel 4a
n.b. the page speed test link also simulates a mid-tier android device
"Enable YouTube pause video when leaving slide (adds extra YouTube Script)" - already unchecked.
To me, it looks like it must be a difference between the android and apple platforms. But i'm not technically qualified to diagnose whats going on...