I have root caused the slow enumeration time to a USB-to-FTDI board connected to the 2 desktop machines with slow enumeration. With that USB connection unplugged completely, enumeration takes ~0.6 seconds (seems to vary per Digilent pod), which is in-line with your results and my testing of ~0.3sec.
But, once this other device is plugged it (but has not sent any traffic yet), the enumeration increases by 2X. Once the other device has been used once (it is necessary for a pre-requisite step to our Digilent application, so removing the connection is not an option), the enumeration time incr