Or search: sleight, celerity, dispatch, facility, rapidity, despatch, instancy, deftness, dexterity, immediacy, adeptness, fleetness, expedition, promptness, speediness
2 Syllables: SU
biggest, bigness, citrus, fitness, fittest, hideous, igneous, mixes, Nicholas, nitrous, piteous, prettiest, quibus, sickness, strictest, strictness, swiftness, thickness, vitreous, witness
3 Syllables: USU
amphibious, assiduous, enigmas, eyewitness, fastidious, insidious, invidious, Mauritius, perfidious, unfitness

Found 30 words across 2 groupings. Skipped 1 excess word of a similar stem.