iPods are slow
Today I learnt that the hard drive in my laptop is fast enough that writing out a lot of data to it is faster than writing out a lot of data while compressing it, but the hard drive in my iPod is slow enough that it is significantly faster to compress on the way in (so that less data is written out). So the point where the addition CPU-time of compression is equal to the saved I/O time is somewhere in between a 4800 RPM 60GB iPod drive and a SATA-2 5400 RPM 80GB MacBook drive