Timekeeping should be taken completely out of the hands of the on-field ref. Just stop the clock for things like injuries and substitutions. And the excessive time taken to restart the game, player 'protests' at free kicks etc. There is then less benefit to feigning injury or running slower than a walk to get off the pitch. Show the time on the scoreboard and stop that too so that everyone knows how much time there is left. The ref can then concentrate on what is going on, on the pitch.