When the internal aggregation was made it only aggregated the content type libraries and not the core components. This was since Core only contained two .js files at the time and one css, resulting in 5 requests per iframe – now there are 13 requests.
Two solution suggestions:
1. Use webpack for the Core JS.
2. Aggregate the files with PHP.
Note that we may not want to bundle the Core JS with the content type's JS to leverage browser caching. (The browser will cache Core JS and reuse it for all content types after seeing one)
Reduce the number of requests per iframe to 4 or less.