There is some loading process (i suppose that are the four videos the chapter contains) and it is not possible to controll which elements get displayed BEFORE and which AFTER this loading process. After the loading process, all missing items get displayed immediately.
Thanks Mackavi for your advise. I tried this, but it doesn't really help. In fact the situation is now as follows:
Items which appeared previosely before loading large videos at first on screen, now - after i delayed there visibility at 0.1 secs - appeared together with the "please be patient, loading video..." textelement (which is NOT initially hidden and should therefore appear at once) AFTER the loading process of videos, leaving the message useless.
No "preload ressources" action of videos is used.
The only item now which gets displayed before freezin' while loding the large video contents, is the defined background image in the page properties. I maybe can use this as workaround, if there is no other way.
Unfortunately video handling in the web-publications is very bad and involving much delays and it seems not to be the loading process which causes this. In reality it are the the show/hide/pause-/play video actions, which in the standard publication works without any perceptible delays but in the web publication each action causes delayes of several secs . The "stop an reset video" action even freezes the web-publication completely while in the standard publication it works also fine. This brought me in trouble and forced me to set up the web publication in an other way seeking workarounds.