What can cause this gup?

Hi

What causes this empty space? between html and css? (in the picture, the red arrow)

https://ibb.co/mfrPMc

Most likely the browser is busy doing something else - could be loading js, parsing huge style sheets, etc. In some cases with some test environments the browser is actually updating itself or plugins.

Take a look at the CPU Utilization graph that appears underneath your waterfall - is that high?