What does speed do to your body/mind?

It’s far from minor. A few few of the things it does physically is cause dry mouth, changes in heart rhythms(your your heart can race and beat unevenly), it elevates your blood pressure and makes you hyperactive.

Psychologically, it does cause euphoria, and increases concentration, but can also make you anxious, nervous and aggressive.

Plus, crashing is just plain awful.

Stimulants increase the amount of neurotransmitters in the brain. The increase i dopamine in particular is responsible for the euphoria that large doses of stimulants produce. The problem is the artificially increased level of dopamine causes the receptors in our brain to need higher and higher amounts of dopamine for us to feel good. Addicts get to the point where they no longer get pleasure from ordinary life experiences and can only feel good with the artificially high levels caused by stimulants. Some individuals need stimulant medications for bona fide medical conditions. At normally prescribed doses stimulants can help a number of conditions at these dosages do not produce the euphoria or addiction that abuse leads to.