What is filesystem.squashfs and why does it take so long to load on to bootable media?

When I use Unetbootin to put a Linux ISO on a USB drive, it proceeds quite quickly until it gets to filesystem.squashfs, which takes longer to process than absolutely everything else combined. Is this writing a new filesystem to the USB, or is it copying some huge filesystem-dependent file? If so, is there a way to only do it once in the event that I will be trying many distros and want to speed this step up?