PageSpeed Insights was the optimize setting. 330x172px
Page speed tests can’t understand what happens with the image in specific scenarios. 330x172px might be the best for the default viewport on larger screens. But since the hover effect zooms in and on mobile devices the column gets 100% width, the image needs to be larger than 330x172px.
I changed 850x443px. Web site opening speed decreased by 1%.
These tests can vary from time to time. This doesn’t have to come from the larger image. Maybe the server wasn’t as fast as the other times or something else. The image is 28 KB large now. It would matter if that would be 200 KB now, but whether it’s 5 KB larger or not doesn’t change anything.