It is very hard to say how fast an engine is unless you are taling about RPM. Looking at an engine by itself the only real speed number you can look at is what RPM a certain engine will spin a certain size prop.

A Cub is probably not the best plane to learn on, not terrible but not great. Cubs tend to have poor ground handling making take-offs more difficult than a trainer. They are generally docile in the air but with their short tail moment they tend to be more pitch sensitive than a trainer as well. Also most people in our club tend to only fly their Cubs when the wind is 8mph or less (which isn't very often).