I have the same issue on my new dell xps 13. I was happily using opera (both developer and stable) on my previous laptop, but after the upgrade scrolling is incredibly slow. I assume this has to do with the high-dpi screen.
This is really a problem, I start to develop a protesting index finger already ;). Is there a way to adjust the scrolling speed somewhere? I was not able to find it in opera:config nor with google.

opera versions used: opera 40 stable and opera developer 42 (42.0.2372.0)
os: Ubuntu 16.10 (x86_64; GNOME)
screen res: 3200x1800

Any help is greatly appreciated (I've been using opera as my main browser for more than 10 years now, but this is so uncomfortable that I am almost tempted to switch to vivaldi or firefox which btw do not have this problem.