Turns out the problems seems to be related to the Nvidia drivers.
By some weird design implementation, the power management algorithm of the drivers decide that when you scroll the screen, or even resize a track, you need another power profile to kick in on the GPU. And, changing the clock profile causes a DPC spike. On older drivers huge spikes happened, now they are smaller, but they still happen and its slow enough to cause audio problems.
So if you just let DPC latency run without moving the screen, everything is great. You will only spot the problem if you open a window and scroll around it.
Unfortunately, there is no way to turn off the power profiles (which Nvidia calls Powermizer) on Nvidia control panel... I found a solution that I'm yet to test:
http://somemorebytes.com/wp/index.ph...e/nvpmmanager/
Why is this only an issue on Reaper I dont know, maybe the other apps use another approach to show the GUI and the drivers do not detect a need to change the profile?