Latest research shows:
"Spraying Speeds - Calculating Tractor Speed, Spray Speed and Coverage
Whether you're spraying with a "Coupe", Air-Blast sprayer, boom or even a hand-gun, you always want to finish the job as fast as possible. Yes
I know
spraying is not the most attractive or comfortable job a grower must do
"
6mph sounds pretty fast to me - unless you want the job done faster
(so laughing)