Porteus

Porteus is built around a modular '.xzm' module system: rather than installing packages that permanently modify the filesystem, extra software ships as compressed modules that get mounted and unmounted on the fly, keeping the core system small while still allowing a large effective software library. It's optimized specifically for extremely fast boot times and running fully from RAM once loaded, similar in spirit to Puppy Linux and Tiny Core but built on a Slackware package base rather than an independent one. It's popular for USB-drive 'carry your OS with you' use cases and for quickly spinning up a known-good, unmodified environment on borrowed hardware.