Fans go wild when screensaver is active - MacBook Pro 2017
My fanspeed also was incredibly high when on OSX stock screensaver. Some virus scans and resetting of the MacBooks PRAM and VRAM later, I did a check and graphed the cpu usage. It was indeed the screensaver process which kept cpu at 100%.
It was set to a transition of pictures in a folder. Apparently the fade and float effect is too much for the mac. Switch to a blank screen with a clock shown. Fan speed went down to non audible since then and CPU is as low as can be during screensaver now.