Which 2009 MBP did you have?I think people too easily succumb to labels. Label something a "Pro" machine vs. something that has an inexpensive base configuration and people label that an "entry level" machine. Actual performance is considered after the labels.
For my development, in many real world usage tests (not benchmarks), disk speed trumps CPU speed. An ultimate 2012 MBA will smoke a machine using a traditional hard drive. My new Mini gets 30-40MB/sec best case reading from disk. Whereas the 2012 11" MBA is getting 450MB/sec. Thats 10x the speed. Makes a huge difference. Compile time is certainly affected by CPU speed, but available RAM is more important to cache headers etc., and a fast SSD. And what about ancillary time that ends up being where more time is spent, such as starting the simulator, starting the app, starting tools such as Instruments.
I do extensive development on a 2010 11" Ultimate. I did many speed comparisons when I bought it compared to my 2009 MBP. The MBP had a much faster CPU, >2x. In some scenarios, strictly compile time was faster. But build and run time was faster on the MBA despite having a very slow CPU. It was able to launch the simulator and kick off the app so much faster that it caught up and passed the MBP for build & run time.
I've been debating upgrading to a 2012 11 Ultimate. It runs 2-3x faster for all tasks than my 2010, but resale value on the 2010 is in the gutter. Folks on eBay don't seem to know the difference between the 2010 base $999 machine and ultimate $1399 machine sometimes paying more for the base than ultimate. For now, my plan is to stick w the 2010. And even it performs ok for most development. I should mention though that I plug into a 28" monitor when at home.
Build times are all about the CPU, and also the amount of available memory. Xcode caches your source files since they are relatively small and the speed of the storage makes very little difference. If you don't believe me, then check out the link below. The guy created a RAM drive (which is even faster than a SSD) and there was very little difference. Sure, a SSD will make things feel snappier, like opening Xcode and iOS simulator for the first time, but once everything is cached, then it's all about the CPU, especially when you have a sizeable project and are running automated tests with your builds (and you should be).
http://macperformanceguide.com/Optimizing-Build.html
Bottom line
(a) So long as the internal hard drive remains fast (not too full), builds will run at top speed without any special effort; a RAM disk (risky in the event of a crash) offers no meaningful help.
(b) External eSATA does not help and a striped RAID actually degrades performance.
(c) Build speed scale closely with CPU clock speed.
Here is another example of what difference a CPU speed makes to build times. This is what happens when all of the other variables are kept constant with different CPUs:
But, having said all that, is an MBA good enough to do iOS development as a freelancer? Yes. Is reducing build times better for your productivity? Yes.