This is illustrated by ZF, describing the 100-300 millisecond shifts of their DCT transmissions.
[1] Reducing shift time is important in performance and racing vehicles because upshifting generally interrupts power delivery to the wheels.
Shift time in a manual gearbox is dependent on the driver, but in automatic or automated manual cars, the electronic or hydraulic control system must be calibrated and tuned to execute fast gear changes.
With a manual transmission, upshift time can be reduced by installing a lighter flywheel.
During an upshift, the engine speed must decrease to synchronize with a higher gear; a lighter flywheel will allow the engine speed to drop more quickly, leading to shorter shift times.