Hello Chris,
when you set the subtrim you are actually moving the servo center. In other words, now the servo is closer to the end point. So the movement of the two servos is not the same, because one is closer to the end point than the other. So the speed will vary, too.

But even with a perfectly identical situation, one servo will move before the other. And this is not a futaba issue, even JR do that, simply because it is a design thing and can't be avoided.

Your TX sends the servo movements to the receiver in a sequential way. So the position are sent in this way: servo 1, servo2, ..., servo n. Given that it is a fact that if we want that servo 1 and servo n move togheter, we can't: there is a lag between the two.

The good news is that this lag is very small with modern equipements, in fact also in pattern airplanes two servo in the elevator are used with success. With earlier systems it was a problem actually in particular in hely setups, where more than two servos must move togeghter to have a straight flying!!!