Hi @stigzler
I hope you are doing well.
This is happening because the LazyLoad forces the image when the page loads/scroll.
The container doesn’t a defined size which will cause this behaviour on your theme.
To fix it, you can contact your theme provider and check for a pre-defined size to avoid this resize, or disable the LazyLoad on your logo, the Logo is the first image that the website renders so there is no problem disabling the feature only for this image.
I checked your website, and I can see there a class custom-logo.
Navigate to Smush > LazyLoad, find the exclusion option and set the .custom-logo on the Classes & IDs option.
If you would like to speed up the LazyLoad script too, you can make it to load from the header, however, this may cause the remove-render blocking warning on Google.
https://monosnap.com/file/PR3ByZkHYv8WEG1IC7kIZgjseWJ04P
Let us know the result you got.
Best Regards
Patrick Freitas