Events sent from other threads are not processed immediately, but are handled similarly to EVT_IDLE events. The main event queue must become empty before they are processed. On MS Windows that won't happen on moves or resizes until you release the mouse.
I see. I suppose there is no possibility to give some higher priority to these custom events, so they would get handled differently than EVT_IDLE events 
Well, as a solution I now do the "forbidden thing" in my program which is accessing the widgets from within my worker thread. Things run smooth this way on MS Windows. The drawback is, that now I have to use different code on Win and Linux, because on Linux accessing the widgets from within the worker thread blow things up. On the contrary the solution with custom events sent to the main thread works well on Linux, even when resizing and moving the window around.
Thanks for the answer,
Markus