Player slows down when hitting an incline

Okay, so the title basically says it all. As my character is moving throughout the world on a flat surface, he moves fine. But as soon as I hit a slope of some sort, he slows down and can't even make it up to the top of the slope.

The slope is 12.5 degrees.

The character is using a box collider (since it's a box right now) with a rigidbody.

Any help is appreciated.