I've never had this issue with prior version but with both the latest update of 4.9.15 & 4.9.17 the UV unwrap feature not working properly.
I'm working with a 30,000 poly model and 3d-coat freezes for like 30 seconds every time I mark a seam or edge loop during UV unwrap.
The issue doesn't happen when I use UV Path features. It has been doing this for hours. I might get a rare snippet of time where I won't get any brief freeze ups.
I mainly work in the retopo room but I tried it in the UV room and it freezes up there too. I normally run GL mode in the because my computer is fast enough but I might try DX mode to test.