For CLI stuff (compiling, file operations etc) it's the time command, for video decode/encode it's built into ffmpeg, and for graphical stuff it's mostly subjective. There's honestly not a ton of difference on most of the CLI stuff since the hardware is the same, but it is measurable. As for the DE, let's just say that Xfce under Slackware and OpenBSD is quick and peppy while Xfce under Debian-based distros is anything but. Ubuntu seemed to be the slowest for that test, and Elementary's Pantheon desktop is a mixed bag. I have considered running the Phoronix test suite for a more accurate result.

Also note that I did have to tweak OpenBSD a little to get it on par with Slackware on the desktop, though the stock install is still faster than the more "modern" Linuxen for most tasks.

And for those who wonder why I do all of this: It's a hobby. It's more fun than watching TV on my off days, and it keeps me up to date on the latest goings-on in the OS world.