over time the plastic hexes can deform and bind on the axels and bearings. when the plastic ones are new and fresh, they are fine. but after cranking down on the wheel nut or crashing a bunch of times, they can actually touch the shields on the bearings causing them to drag which hinders top speed and causes uneeded motor heat. I once rebuilt my drivetrain 2 times wondering why it wasn't as free as other people's minis (I'd see it on the straight using matched 15k motors, even swapped motors for 1 qualifier). It turned out to be the dang gone plastic wheel nuts. changed them to the aluminum hexes and never had the problem again.