I've recently bought a Macbook Air and I'm using a 64GB SD card to store my iTunes library there, so iTunes is constantly reading from the SD Card. The first times I used it I was quite disappointed, as there was a very important (1-2s) delay when switching between songs, which doesn't happen for example in other computers or my car which has an SD reader slot. However, the other day for no aparent reason the SD card was behaving incredibly fast, switching songs was instantaneous, but today I'm back to the old slow speeds.
Is there some reason why the SD card is fast or slow to read in different scenarios? I haven't plugged in anything to the USB port that could be hogging the USB bus.