We’ve seen that some browsers may have an upper limit on how many iframe elements they will handle in a single page (Safari used to stop at 500-something, for a while). It’s possible that you’re hitting something like that. (We try to use Shadow DOM containers in browsers that support them, but iframe is pretty prevalent still.)
Can you link us to an example page so we can debug the source of the error?
Another option for displaying that many Tweets would be to use the Collections API to create the set of Tweets on the backend, and then an embedded timeline to display them. It’s a slightly different display, but could be much more efficient for you.
Ben