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As fast as the harpsichord soloist can play the runs cleanly and musically.
A concerto usually has three movements and involves a soloist an an orchestra.
Trill
cadenza
Almost never. The main exception is for a soloist during a cadenza in a concerto.
I believe the answer you are looking for is concerto - a work for orchestra and soloist.
Canon in D
Concerto Grosso
The extended virtuosic section for a soloist in a concerto is called a cadenza.
A work for orchestra and solo instrument is called a concerto.
There are many ways a soloist can display his virtuosity to the fullest. Most often this happens during the cadenza of a concerto.
Cadenza