Looks like this:
What type of mouse do you have? I have a sort of newer Razer Deathadder mouse and it works just fine. Might be a driver issue with it? The mouse itself can be adjusted for sensitivity as well. But, I am sure more optimizations in settings are coming before full release for mouse look and other options. I know for sure there are buttons on this mouse that change the mouse speed....every time my granddaughter comes over and I use the mouse, it is all jacked up from her pushing the buttons, lol.
There is also a specific mouse sensitivity setting in the game settings you can access ingame. On one of the tabs.
Brand new Razer Basilisk. Some Logitech thing prior to that. I'm 100% sure it's at least related to how I have my speed set in Windows, but as I said, this isn't a problem in any other game. They all have enough adjustment to deal with it. I'd really rather not have to change my mouse speed settings every time I play Pantheon, and this seems like an easily solvable problem.