I just tested this with Corona V4 Hotfix 1 and 3ds Max 2020.
I followed this guide:
https://forum.corona-renderer.com/index.php?topic=11630.0Everything works fine here.
Here are my notes:
1) Nothing happens when I drag-and-drop the script into the viewport, but I can find it in Customize user interface > Toolbars > #PankovScripts , then just drag-and-drop into the UI
2) The first time I clicked "Create IES to..." it said "Operation canceled" and did not proceed with rendering. So I just pressed the button again, and it worked. I think it just needs to "fill" the empty Max VFB with anything before rendering.
3) I can confirm the slowdown with IES in V4, but this happens only with this custom IES file. Regular IES shipped with Corona work fine. I guess it's because of the complexity of this custom IES. You can compare them by opening them in notepad.
Conclusion: everything is fine and works with Corona 4 in MAx 2020.