I like Speed Kings well enough. I tend to always have one in use. I have worn out about a half dozen over the years, so I feel qualified to state that their sturdiness is modest. These days replacement parts, and repair wisdom are both much more available than for much of the Speed Kings history.They are fast, but their action is different because of the compression springs. This particular feature is probably the one that determines whether you are a Speed King fan.I also use a Caroline (wonderful), Yammie Direct Drive Flying Dragon (also wonderful), Capelle (more cool than great) and a Tama/Camco which is more to my liking than the Speed King--though no more sturdy.
I'm not sure why a compression spring would be any different than any other spring. There are at least 6 different springs on pedals: compression coil, extension coil,combination coil, torsion leaf,concentric leaf and leaf. There is even a rubber band pedal. Many of the extension coil spring models use poor springs, giving a false view of the capability of that design but in the final analysis a spring is a spring, and as long as it is designed to retain it's potential energy ,even after extreme deformity , one is as good as the other.