"Fast" is a relative term for most of us, probably based on out individual tolerance for how much we're willing to spend on "speed." But in general, RAM and processing power will have the greatest impact on performance when it comes to responsiveness.
I'm primarily a LR 5 user, with Aperture in the mix when I need it.
According to the internet, multicore CPUs
do not appear to positively impact LR performance when you are making edits (Preview Rendering). Having not used a powerful single-core then a multicore setup back to back, I can't confirm or deny this claim personally, but some very smart-sounding people with some very convincing looking benchmarks have made this claim. Just give it a google, here's a what I found:
http://www.slrlounge.com/lightroom-lr5-lr4-hardware-performance-test-review
It appears that LR's Develop mode doesn't necessarily benefit from multicore CPUs (the "bottleneck" being waiting for a settings change to render as a preview), and the same goes for image export.
Take all of this benchmarking with a grain of salt though; Adobe claims that LR supports multicore processing. Of course, Adobe says that their LR software can
use multiple cores, but Adobe doesn't say that LR actually
benefits from being used on a multicore system. Don't you just love marketing spin?
Here are a few tips for optimizing LR, I gave them a try and yes, they did help:
http://helpx.adobe.com/lightroom/kb/optimize-performance-lightroom.html
Buying as much RAM as possible, increasing LR or your image editor of choice's disk cache size, and the use of SSDs instead of hard drives will also help, perhaps more than buying the fastest multicore system you can find. If you're keeping your system for longer though, I'd buy a solid multicore system. I imagine it is only a matter of time till all software makers find ways to make better use of multithreading.
Purchase the best you can afford and it will be faster. A lesson that is as old as the computer itself. Ha.
