The act of speeding just after escaping from an area of dense traffic. Flooring it after you pass the car in front of you often leading to speeding tickets or warnings.
"Jim got pulled over yesterday for freedom speeding on the freeway."
"Freakin cop pulled me over when i was only freedom speeding for ten seconds."
When you're walking/riding/driving so slow that you get aggraveted by the situation, so you say "I'm speeding like a plum!". Since plums can't speed at all the direct meaning is contrasted in a sarcastic definition, emphasized as going so slow, almost stopping.
(carrying Corey in the bike)
Corey: Hurry up Tim! We're late for the soccer game!
Tim: I can't! I'm doing the best I can but You're too heavy.. we are speeding like a plum!