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 sarcasticdefinition, 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!
1) A spelling beethat the participants are very young, namely kids in kindergarten and 1st grade. The age of the participants often result in funny misspelllings.