I have to ask, did anything else change at the same time? Although the i7 920 is a fearsome chip, and a great upgrade over the older 65nm Core 2 Quads, anything approaching even 2x performance per clock I'd probably consider an edge case. Sounds like there was probably another variable in there.
I think a lot of it comes down to hyperthreading, actually. The 3x improvement was in transcoding, which is a reasonably pure test of CPU and memory throughput, rather than amount of memory (4GB -> 12GB), or I/O speed (the transcoding was actually done with both input and output across a gigabit network to my NAS).
Nailing down the improvement in build time to any single change would take more time substituting things than I'd care to expend right now. The extra memory for file cache no doubt helped quite a bit, as did the SSD, but different parts of the build are I/O heavy, and other bits are CPU dominated. My MacBook Air 13" (4GB RAM, SSD etc., but slow processor, running Windows 7) isn't particularly fast at the build.
