Exporting using Imovie
I have a 2014 mac mini ($699) with 2.6 dc i5 1Tb 5400rpm, saw your post and tried it out using imovie, I do have lightroom 3, but didn't try that.
I loaded two projects, one was 2Gb shot with a GoPro in 720p, the other was 20Gb shot using a Sony HD Cam in 1080i. Using share feature, I exported both as a "file" onto the desktop as 1080p set at proRes quality.
Under no load (no other programs open, just Active monitor and Imovie)
2GB file took 1min 15sec
20GB file took 17min 10sec
Under load (had Safari with 4 tabs, Iphoto, Mail, Itunes playing music)
2GB file took 1min 30 sec
20GB file took 18min 25 sec
The weird thing was that using a memory free program, it showed all 8GB of memory were being used under full load, yet using Activity Monitor it never showed memory pressure (stayed in green whole time). Although, looking at CPU usage, it fluctuated between 70-100%, with most of the time sitting in the 90's. Doing other processes, like surfing Safari, did seem a little slower, but considering all the stuff I had open, it was actually not that bad.
The difference in time is really small, I think with an SSD HD instead, this would have been sped up a ton. Not sure how much faster it would have been on the 2012 2.3 qci7, but I really feel the bottleneck was the 5400rpm stock drive. just my $.02