Animation blending seems non-linear

I’m trying to get some basic character animations working.
The character can move in any direction, with a walk speed of 4, and a sprint speed of 6.

Now, running forward at 6 units, the animation is as expected. While at 4 units, its very slow. Way slower than the ~50% it should be.

Here is a video of how it looks.

I’m assuming there is some weird interpolation going on, but no idea.