I asked earlier but my question seems to have disappeared.
I tested this with 100% reproducible results
OS versions tested Windows 10 - 1703 / 1903
Clean installs and no other apps running except Vegas and Task Manager
With the stock MS Driver
Time to render 6:20
With the AMD drivers 19.3.2-Mar14 and 19.9.1-Sep4
Time to render 9:20
Almost 33% slower.
Another issue is with the stock driver Davinci Resolve won't open. So either I'm stuck with poor performance and can use Davinci and Vegas or I have good performance but can't use Davinci.
It's as if the official driver from AMD is throttling GPU. Temps are all low though and a well ventilated chassis with mostly NVMe / SSD's storage.