TapNoiseTo change how a held key repeats, open System Settings › Keyboard. Delay until repeat controls the wait before repeating; Key repeat rate controls the pace after it starts.
Tune the delay first
Open an empty TextEdit document. Hold Delete after typing a disposable sentence, and notice when deletion starts repeating. Increase the delay if a slightly long press removes more than you intended.
Then change the rate and repeat the test. A faster rate may help with navigation or deletion, but it can also make overshooting easier. Change one slider at a time so the result is understandable.
Repetition is not every duplicate-letter problem
These controls affect keys you keep held down. If one brief tap consistently creates two characters, test another app and another keyboard before assuming the repeat rate is responsible. A hardware or connection problem needs a different fix.
Some letters show an accent picker when held, rather than a row of repeated letters. That is normal text-entry behaviour in supporting apps. Test Delete or an arrow key if you specifically want to observe repeat timing.
Sound apps can behave differently
TapNoise deliberately ignores automatic key-repeat events for ordinary key sounds, so holding a key is not a reliable sound-volume test. Test distinct presses instead.
If even a short tap takes time to appear, inspect Slow Keys. Its acceptance delay is a different setting, under Accessibility, and should not be confused with repeat timing.
Source: Apple: set how quickly a key repeats, TapNoise sound behaviour.