If the sensor and magnet are correctly installed, the wired computer will be virtually100% accurate at counting wheel revolutions. Converting wheel revolutions to distance, measuring time, calculating speed from time & distance, then displaying speed on the screen, are matters of calibration, software, and hardware. There may be calculation inaccuracies and delays updating the display. Generally I think most cyclocomputers are pretty good, but they are not all equal.

Theoretically, if well calibrated, this should be far more precise than a GPS at both instantaneous speed and speed and distance over a long ride.