I found the solution. You can't plug the microphone into the front USB ports. The boot time went back to normal when I plugged the microphone into the rear USB ports. 
I recently purchased a HyperX Solocast mic for various purposes and I've noticed that when it's plugged in my boot time takes at least 2 times as long. What could be causing this?
XPS 8930, HyperX microphone increases boot time