If you are practicing bunt plays at a slower speed than the
fastest runners, you are practicing to fail.
It’s very important to practice the timing of bunt plays so that players get to experience game-like conditions. Usually batters/runners are used so that the fielders get used to trying to beat a real-live runner heading to first base. One problem with this is that the runners chosen may not be the fastest kids – the fastest may be the infielders who need to be practicing the bunts. Even when fast runners are used, they tend to get tired after a few sprints.
To combat this, a great tool to use on these bunt plays is a stopwatch. The first thing to do is time the fastest runner on the team from home plate to first base. Have a guy bunt a ball and start the stopwatch as soon as the ball makes contact with the bat. Let’s say his time is an even 4 seconds. After players are taught the proper footwork and are allowed a few reps at a slower speed to get the hang of things, bring out the stopwatch again. Tell the players that every bunt play must be completed in under 4 seconds. Better yet, take the best time to first base you’ve ever seen at your level from any player. Every bunt play you do in practice must beat that time. When the play is made at first base, yell "Safe!" or "Out!" depending on the time. It can be quite an eye opener for players. It clearly sends the message that quickness to the ball and properly setting your feet/body prior to fielding it are key.
... and a stopwatch doesn't get tired.