It’s a Unity, or generally, a physics thing.
The proper solution, where the speed is maintained on slopes is advanced even in script, and having moved on from Playmaker, I do not know if actions exist to make it. You would shoot a ray down onto the slope, get the normal, use the vector perpendicular function to get the perpendicular angle (i.e. the angle to move) and set velocity in that direction.
However, the “proper” solution is often not needed or even wanted. If you just want to climb slopes at some lower speed, you detect the slope and get the normal, run through vector angle to get the angle, and if it exceeds some value, tweak gravityscale, drag and your velocity/force while on the slope.