In an attempt to relieve pressure on the hard drive of an older Microsoft Windows Vista laptop, I formatted a one gigabyte SD card using FAT (exFAT is not available on Windows Vista). I dropped the SD card into the slot, right-click in File Explorer and dedicated the SD card to ReadyBoost. The laptop immediately began to feel sluggish and shutdown took many minutes whereas it had been quick. The Windows Vista version was Home Premium.
Some exploration and looking at running processes led me to find that Windows was trying to be helpful and preparing to sync files from the laptop to the SD card. This entailed periodically (seemingly every few minutes) starting mobsync and wmplayer.exe. The SD card was entirely dedicated to ReadyBoost, so there was no possibility of syncing to the card. I removed the SD card, mobsync and wmplayer stopped running, and the machine was again very responsive and shutdown was quick.
I recommend ReadyBoost when the primary drive is not speedy. Flash memory on a SD card or USB stick, particularly when running through USB 3, will relieve pressure from the hard drive. But be wary about doing this on machines with Windows Vista which may then start other processes and actually degrade performance.