If I may....
SPPB 4 Beta 2 is already published, if we talk about RC versions - April, next weeks.
Stable version - probably May 2022.
"The header, footer can be designed with Elementor and are automatically used on every page."
Yes, but have you checked how many requests it makes to the database and why WP sites are so heavy.
Besides, even now SPPB Module can be used as a Bottom and Footer, even we use it this way.
About header also possible, I have seen it on our user's sites in the past. But I agree that many things can be improved to speed up the website building process