Hello Frank,
I know it's been a long time since you have post this issue, but at the moment I am getting the same problem with very similar combination.
Wanted to check if maybe you have found a solution.
MBD X299 WS SAGE, VGA RTX2080, CPU i7 7820X and 16GB or RAM with Windows 10 Pro 64bit.
System is very fast and quick responsive up until I have inserted Decklink Quad2.
Even without drivers installed, system got much more slower.
Tried with several version of drivers, nothing has changed to better.
Tried also to install Decklink Quad2 to serveral of the PCIe ports, also no changes.
MBD BIOS version is 1001.