New finding:
Well, obviously it is not the app itself, it ist the graphic.
While diplaying the windmap, you can use the power indication as a countdown. Opening Metar or any other text, everything is okay. Maybe this is helpful?
Is my smartphone the only one concerning this issue?

Dear Windy-folks,

I like your App very much, it is very useful in preflight an flight. But on my lg g5 se there is an overwhelming power consumption as shown in the attached screenshot while using the app for not a minute. Might be there any solution except to purchase another smartphone?
This issue doesn't occur on my samsung tablet. I will try it on some other smartphones.