They are not at ballistic missile speed but they are not that slow. After launch they cruise at commercial airline speeds - 800-900kph. This is a similar speed of other long range cruise missles. They are very, very, large (14m long) so they are easy to see and film from a long distance. Their size is the reason why they are relatively easy for anti-air to detect, lock-on, and shoot them down. When they do make it through and hit the objective, it does a devastating amount of damage.

For comparison the Tomahawk missile flies at a similar speed and height but is only 6.25m long. It also packs 1/2 as much explosive.