I've just loaded your attached profile.
All the velocity / steps / acceleration settings are the same so I can't really see what might be causing it.

Are the slow motors missing steps? Try sending a slow motor one turn forwards then one back and check it returns to the same point. It might be that the driver board is 'missing' lots of the pulses making the motors go slow.

Try going to config->motor tuning and putting the velocity setting up for the slow motors and see if it makes a difference.