Some distributions of the open-source operating system are bloated, but Puppy Linux, Knoppix, SLAX, NimbleX, and — yes — Damn Small Linux will get you up and running fast.
Enjoy the reviews and overview here
When you boot DSL (and it boots fast), one of the many boot-time options you can select is to load the whole distribution into RAM for the sake of speed. Because DSL fits in only around 50 Mbytes, this is one of the few distributions where you can get away with doing this on virtually any machine. I did some testing and found that you really need 96 Mbytes of RAM or more to run DSL entirely in memory; anything less than that and you need to run from the boot media.