Hi! I’ve looked at your website and I think the issue is not with the cookie banner …
I think the issue is similar to your no. 6 (Content reflow) from here: Performance issues
From what I’ve seen the problem is with a CSS variable --spacer-height that initially has a default value (105px) and then it is adjusted using when the page finish loading.
See more details here, where it was discussed: --spacer-height bug when I edit the header
Hope this helps!