In timed sports, this is exactly what you do when you are ahead. We aren't used to tennis being a timed sport. It changes the entire dynamic of the game.
Some people will think it is smart to use every second allowed if they are trying to run out the clock. Going beyond this is cheating as Cindy said. It is no different that hooking on line calls.
Everything comes back to players acting as their own officials. If you feel someone is cheating, a grievance is really your only option. It is probably not to get the results voided, but to get it on the other players record. I would guess the problem would disappear if they get several of them.