I checked this, and it's really odd.
In the file sent by Jvincentrs4 the Corona bitmap is super slow indeed. Just plug it into 3ds max enviro and change rotation. It looks like it's re-loading the file from disk each time this is done. In this file, if I load some different hdri file, it will be also very slow.
BUT if I create a new file, it's super smooth, even if I use exactly the same hdri file that is used in the reported scene.
This makes me believe it is related to the scene itself, maybe to the version of Corona in which it was made. It looks like it's 08.02 daily build. I am using the newest one (
https://coronarenderer.freshdesk.com/support/solutions/articles/5000570015).
Jvincentrs4, can you check if it's also slow for you if you create a new file using the newest build?
And I will check what happens if I save something using the older one.
Update: the slowdown is there only if the bitmap is loaded using 3ds max enviro, not when loaded into Scene > Use Corona
:S
Update 2: I tried hard, but I am not able to reproduce this, regardless of version saved and version rendered.