Ignoring the fact that basically all drug development outside of the computational stuff is large amounts of trial and error, there is ample evidence that ADHD results from dysfunction of specific neural pathways, and stimulant medication benefits ADHD individuals (more than just "speed helps everyone focus").

> This literature is increasingly contributing to the notion that the pathophysiology of ADHD reflects abnormal interplay among large-scale brain circuits. Moreover, recent studies have begun to illuminate the mechanisms of action of pharmacological treatments.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3876939/