Stormraider Bug

Im not sure if anyone already reported on this or how long this has maybe been a "bug" but I noticed it doesnt give you anywhere near 40% speed, the slow resistance works though, for sure. However, when I activated this, I only got 10 movement speed flat, no where near 40% and ive done the math to see how much you're suppose to get for it to be 40% with boots of speed. ( the 25+ movement)