Why is Adobe Media Encoder soooo sloooow?

I have a 2.5 hour corporate event that I edited in Premiere Pro. There are AE clips, sound, imported video, the works. When I run it out to AME, the source resolution is 1440×1080 and I am downsampling to 640×480 for delivery on Vimeo.

The thing is… Media Encoder is teling me that it is going to take 20 hours to render this video. Why would it take that long? I rendered last night at full resolution, it took 8 hours and produced a 27 gig MP4. I have run out video this size (previous events) with way less of a time investment. Clearly I am doing something wrong! My newbie-ness is showing something bad here.

I am on a Mac Pro, 2×2.4 GHz Quad-Core Inel Xenon tower with 22 GB of RAM running OS 10.8.4 and the Adobe Master Suite 6.

Can you make a suggestion, or maybe point me at a tutorial on render settings? I thought I had everything set right, but this is crazy!