My mother has a Bosch 1400rpm machine, which she always sets to 1200rpm. It has a tendency to vibrate quite a bit at 1200. Out of interest, I washed a load and spun it at 1400rpm: it went through 1200 with a bit of vibration, but became the most stable machine when spinning at full tilt.
I've noticed this before with other machines; at certain speeds, the machine and/or suspended 'wooden' floor, oscillate at a natural frequency - usually around the 900-1200rpm speeds. Faster spinners break this frequency. My Zanussi vibrates a bit at 900rpm (it has a staged spin profile), but becomes more stable at 1200, and rock solid at 1600rpm.
Faster speeds may wear out cheaply made drum bearings in a shorter time span.