I should have qualified extremely fast: From what I hear, it can make compiling go faster than the MBP, despite the slower processor, solely because disk reading is so much faster. With a large project and lots of small files, disk access adds up.
If your source files are located contiguously on the hard drive [1] and writes are cached to occur in big blocks, while also using some compile-in-memory techniques like those outlined above, I imagine compile times would not vary much on flash-disk MBAs as compared to those with conventional laptop drives.
[1] This could be accomplished by keeping your project in a compressed file and loopback-mounting it to your project's working directory.