When I experienced extremely long bootup times, this solution worked for me:
In terminal, edit the file /etc/init/failsafe.conf (e.g. sudo pluma /etc/init/failsafe.conf)
Then comment out sleep commands by adding the pound sign (#) at the beginning of each of the four lines involving sleep.
My bootup time was hanging on the comments "Waiting for network configuration" & "Waiting up to 60 more seconds for network configuration." So, your situation is likely different. But, no pain to try.
By the way, after dealing with some Network Manager fixes, I no longer have to use this method.
ASUS U56E 64-bit Mint 17 MATE Qiana dualboot WIN 7
Zoom #3095 dialup USB modem