Is Spigot more lightweight (in terms of memory and cpu) than a vanilla Minecraft server?
From the About page it seems like it has lower idle CPU levels, optimises some of the memory uses, and heavily reduces network usage. It sounds like it uses its CPU cycles better, but doesn't necessarily reduce them when it's not idle, in order to give higher performance at the same CPU usage.
Apart from guesses from the About page though, you can only really try it and compare. If you do, you can post an answer to your own question.
Having worked with spigot for a few years, as well as server development in general, technically, spigot is not as lightweight as a vanilla minecraft server.
All server mods and versions you see are built on top of the vanilla minecraft classes. Developers generally use a minecraft Mod Coder Pack. These different server versions are compiled with vanilla resources all together to create the modified server versions you know and love. That being said, I believe spigot still uses its resources wisely.