The exact changes a version, including v12 RC1, brings is listed in the [changelog](https://forum.corona-renderer.com/ Chaos Corona 12 for 3ds Max - Daily Builds Changelog ).
The only remaning, known as of today, slowdown when “Loading Corona assets” is displayed is related to cloth/garmet maker in 3ds Max and has nothing to do with Corona.
An investigation done yesterday has confirmed this. Corona is just unlucky that during this long loading Corona assets is written.