I have never heard of any procedures or airways that require a minimum airspeed. Having a minimum climb gradient is quite common, typically on departure procedures to ensure terrain clearance.
It generally works out in such a way that aircraft with similar performance tend to cruise at the same levels. Most passenger jets cruising around 30,000-40,000 feet all happen to have a cruise speed of somewhere in the mach .75-.85 range. Slower turboprops often cruise at about 25,000 feet, and below that various GA planes. Granted, there can be quite a speed difference for those planes cruising at about 25,000 feet and below (for example, an ATR 72 has a cruising speed of 275 KTAS, a Dash 8 Q400 is about 370 KTAS - they both commonly cruise at 25,000 feet). However, these levels are typically less crowded, so it is not so much of a problem. For the really crowded, higher levels, most aircraft have very similar performance.