Instructions
Increase the speed in which your horse responds by giving it a pre-cue. For example, when you say "ho" and then pull back on the reigns, it will stop. With practice, you only need to say "ho" for it to stop.
Take your legs off the sides of the horse when you are sitting on it. Before you kick it to get the horse moving, say, "That meant move." It will eventually move when you take your legs off its sides. This will make it move faster in the future because when you actually do bring your legs back and kick it, the horse will interpret that as it isn't moving fast enough.
Take your horse to the vet to see if any physical ailments are making it slow. Include in the exam a blood analysis to see if any diseases are present.
Wait for the weather to cool down. Many horses are less active when the weather is too hot.
Ride your horse vigorously for no more than six weeks -- anything more will burn it out and cause it to become sluggish.