is the nx200 UHS-I compatable?

You should definitely update the nx200 firmware. It takes me about 5-6 sec to write 3 shots using ScanDisk Extreme and about 7-8 sec using Samsung class 10 card.

And if I do 8 shot burst (it says 7 in the menu but actually takes 8 shots) I need to wait about 21 sec for a green light to stop flashing when using ScanDisk and about 4-5 sec more using Samsung card

Ive just tested the 8 GB Transcend class 10 card:

12s for a burst of 3 shoots and 36s for 8 shoots burst