I do not currently have any employees, but here is an idea. What if you assigned a point system to the dollar value you charge. Say the points are base one dividing the price per lawn by 10 to keep it simple.
For example if you have a $40 lawn you dived the 40 by 10. And that lawn has a point value of 4. Now make that point value known to all employees. Now you can pay all your employees $10 an hour and then when they get raises just raise their point percentage. Example: Your crew leader starts at $10 per hour plus %100 dollar value of the points. So if his crew completed 2 $40 lawns in one hour he would make $18 further more the crew members would make $10 plus %25 the dollar value, so they would make $12 in the same hour.
This both keeps them working quickly, but also efficiently, because if they have to return the following day they don't receive any more points.
Just my thoughts.