I Am 60 Years Old, Good Health Other Than Elevated

I am 60 years old, good health other than elevated blood pressure. I take know medication other than 15 mg of dextroamp-amphetamin in the morning for better focus at work. When I ride my trail bike at high speed my vision gets very blurry. If I ride at slow or medium speed my vision is fine. I would like to know what could be causing the blurry vision at high speed? Thank you