Suspension — the series
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A tied note is the oldest thing in its own dissonance
A suspension is taught as a dissonance that resolves, and the sum is done on a chord symbol with no time in it. Put the time back. On voices or an organ, a 4–3 is no smoother after its step than before: of eight spacings, two fall and four rise. On a piano the tied note was struck a chord ago, so it supplies under a third of the roughness it is blamed for, the fall to the resolution is the clock's, and the step itself makes the sound about three times rougher than leaving the note where it was.
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A trill is half a consonance unless it borrows its own resolution
A struck suspension loses roughness because its tied note is old, and the obvious repairs are all things a player already has. Three of them fail in ways nobody would predict. A re-strike under about nine decibels below the chord is smoother than the tie it replaces. No rate of trill stops the fading, because the three notes under the trill are what is fading. And a trill on the upper neighbour spends half its time on a chord note: only the trill that alternates a suspension with its own resolution gives back more than a single strike.