He slows down because he's not trying to go up the slope, he's trying to go straight and the slope is pushin him up.
I'm a noob, but I think AlignToVector might help you if you align the character to the slope so that he's moving in the direction you want at the normal speed. The upside is that method doesn't involve math, but the downside is that - unless your character is an ant - it will look bad because if the character is climbing a 70 degree mountain, his body will be almost parallel to the ground!
Using math and such, I think you can use some of the collision normals functions to find the slope of the hill and head him in that direction.
I'm a noob, but I think AlignToVector might help you if you align the character to the slope so that he's moving in the direction you want at the normal speed. The upside is that method doesn't involve math, but the downside is that - unless your character is an ant - it will look bad because if the character is climbing a 70 degree mountain, his body will be almost parallel to the ground!
Using math and such, I think you can use some of the collision normals functions to find the slope of the hill and head him in that direction.