Page loading slowly and showing blank before ready


My webflow site is loading quite slow, and I noticed it always flashes blank before it is ready. This makes the FCP=LCP, but both way too high.
I’m using quite some custom code for analytics purposes etc., but also when removing or blocking them it stays similar.

Here you can see a screenshot of what I mean:

Would anyone be able to point me in the right direction?
This post and this one didn’t really help.

Read-only link here.

Thanks a lot!