I just picked Alpine Linux for security and efficiency built in mind out of the box. It still required a lot of configuration, for lowering attack surface and hardening.
I know you asked for a lightweight distro, but from a security standpoint, I recommend using whatever allows you to relock the bootloader. Which looking up I dont see any choice. If relocking doesnt matter, then any distro for speed is basically any distro that comes with as minimal stuff as possible.
Not speed so much as disk space, non-upgradeable 16g emmc severely limits the use of them. I have like 3-4 chromebooks I'd like something small on. Most distros get up to 4-7 gb once updated and equipped with desktop and browser. I'd like to see a loss of 1.5-2gb for a basic desktop.
It certainly makes great VMs - graphical or otherwise. I think some people use it as a super lightweight distro as well, though my attempt to install it on a less powerful PC failed (probably a firmware issue).