Is something changing your speed variable? To find out, make it a const and see if the problem goes away.
Also, it’s difficult to see code in a video, so instead please use code tags: Using code tags properly
Generally, to help gain more insight into your problem, I recommend liberally sprinkling Debug.Log() statements through your code to display information in realtime.
Doing this should help you answer these types of questions:
- is this code even running? which parts are running? how often does it run?
- what are the values of the variables involved? Are they initialized?
Knowing this information will help you reason about the behavior you are seeing.