@OP just pick one. They are all pretty much the same. (all mostly based on the same linux kernel)
If you want something fast then go for the most stripped down version of -insert linux distro you choose-.
KDE versions are heavier distros and for older cpu not recommended. XFCE is probably one of the lighter weight distros and will seem to operate faster on older hardware. Same goes for mint/cinnamon distros. As far as most developed distro goes ubuntu is probably the most stable.
I prefer debian myself. I just get the server end and build my desktop environment. It really doesn't matter which version of ubuntu you get as you can just download a different environment from any of them to try out. (XFCE, Lubuntu, KDE...ect)
Personally I like one with nothing so I can build and download what suits my needs.
I use Kali Linux and Ubuntu. Ubuntu is for my website and Kali Linux for my... certain needs.
Lol, Kali is a pretty good looking desktop environment from debian linux. (that's about all it is) It's loaded with outdated (AKA already patched) scripted for pen testing. Kali was built to give you general already heavily publicized exploitation tools. It is good for mom and pop sites to pen test their website to make sure the general exploits wont allow hackers to gain access/ shut down site.
I swear everyone who
downloads Kali thinks they are some great hacker.
Kali as a default distro really sucks as it is not too stable. Hell, half of the "tools" it comes with don't properly work.
They really should have charged when they released this distro. All the kiddies would have paid to download it so they could flex their leetness.