I might have found a possible workaround for the high GPU load during to update (launch) screen:
As I wrote earlier minimising the window (that is, clicking on the “-” button in the window title bar, top right) does not seem to reduce the GPU usage at all (the application should detect that it is not visible on the screen and should hence stop updating / rendering its window content completely).
BUT: resizing the window until it becomes as small as possible seems to reduce the problem to a 30% GPU usage. Still way too high, but at least the coolers are shutting down every then and when.
There is hope that this update (which is being downloaded) might fix this issue, but given the fact that it has been reported since ever makes it unlikely still (it is a “world update”, after all).