I can remember when I got my first war steed I didn't have this problem. It was only when I put points in the skill tree that increases the steed's speed that I first saw the slow down/speed up issue. At that time you had to increase the speed first in order to improve your maneuverability. As it is now, if I run my war steed at full speed in a straight line it's fine. But if I try to change direction or circle something the problem starts up. I always wished they would just decrease the speed of the war steeds. It may not totally fix things but I think it would help.