#1: on latest NVIDIA driver 610.47
open CRU (Custom Resolution Utility)
Go to CTA-861 Extension Block
Leave only Default RGB and sRGB enabled
Use restart64 or Reboot.
After doing this, Alt-Tab became almost instant with no black screen delay.
#2:
Install NVIDIA driver 576.88 or older.
With 576.88, the problem doesn't occur even with the monitor EDID left completely default.
Not sure if this is a monitor EDID/CTA-861 issue or a newer NVIDIA driver bug, but hopefully this help someone else experiencing the same problem.
Let me know if you can reproduce it and whether you know why this happens.
