This only occured in the OS version 188.8.131.52. When a note is pressed on a conventional MIDI keyboard, a MIDI message is immediately transmitted out of the MIDI out port. This is not the case with a USB MIDI interface. Instead of a message being sent as soon as the message is triggered, the message is queued in memory called a MIDI buffer within the X-Station and the computer collects the message when it is ready. This is done by making use of the USB MIDI driver.
A Windows PC loads the USB MIDI driver ready for use as soon as the USB MIDI interface is connected. Mac OSX on the other hand only loads a USB MIDI driver ready for use when an application that could potentially use the driver is opened!
Consequently when the X-Station is powered by USB from an OSX Mac, if a music application is not in use, the MIDI driver will not be loaded. Since the X-Station is connected via USB, generated MIDI data is sent to the MIDI buffer but this is not serviced by the USB MIDI driver. The MIDI buffer is of a large, but finite size and when this is full the X-Station will respond to new MIDI events very slowly.
There are three solutions: 1) update the OS of the X-Station. 2) Open a music application or even Audio MIDI Setup (AMS) that can be found under applications/utilities. This is enough to load the MIDI driver and normal operation will resume. 3) from the template common menu, stop the X-Station from transmitting data to USB when the USB connection is only used as a power supply.