Well yeah, that’s why I said, you can change the max walkable slope when the condition is right. So on tick, do a check to see if the character’s velocity is greater than X value, and when it is, set the max walkable slope to something goofy like 360, and when it’s not true, set it back to whatever you want your walkable slope value to be.
You’re soon to discover that physics in a game engine are not the same thing as real world, and that although it may look like your character isn’t walking, if the capsule is touching the ground, the engine considers that walking so he stops at the point in which the capsule comes into contact with the slope and it exceeds your slope angle which is exactly what you told it to do 
And during testing, see if you can get the result you want by screwing with the numbers and then when you get it working, you can worry about things like setting the max slope angle when the character isn’t moving fast enough.
You need to specify the conditions under which you want the behavior to change, the engine isn’t going to figure it out for you.