We're currently building a web application with ArcGIS Experience Builder and beginning to encounter an issue where the users web browser crashes with the error "Not Enough Memory".
The Web Experience we are building comprises of 6 pages, and the majority of the pages includes a map accompanied by a data centric widget i.e. a list, feature info or edit widget. We've configured the maps on each page to synchronise their zoom extents, and we've also applied data filtering actions throughout the app. Additionally, one page consists of an embedded Survey123 form, which has 3 pages, with 2 related tables.
All in all, its a relatively complex app with a lot of content - but this is exactly why we want to use Experience Builder.
I can't share links or screenshots unfortunately.
I had issues running out of memory last week too. It was because I had a layer added in through a web service, but that's probably not your issue.
In regard to number 2, in the past I've had issues loading because I've had too many sections, some of which contained S123s, but then they added in the lazy load check box and that sorted things for me.
These are some small things that might help, but I'll keep an eye on your thread as I'm curious about this too.