White space scroll lag in parallax images

Hi there

I’m trying to set up a website with your theme but I’m experiencing some issue with parallax background images during the scroll up and down.

In the screenshot you can see the white space that appears during the scroll. Not a big issue in this screenshot (that’s the first one i got to show you the issue) but is very annoying when a parallax background is set after a darker area.

more info:
I’ve set the parallax images as a line background (not widget). Same issue both with child theme and the original one.

Can you suggest some fix?

Thank you in advance