Storing images on a sub-domain
 

I stumbled across an article that implied that storing images on a sub-domain and linking to them on your main site may improve your SEO performance because pages on the main site load faster.

Do any adult webmasters here do this or do you think it really doesn't matter?

You're probably referring to ravelrumba.com/blog/static-cookieless-domain/.

It's an end-of-the-road performance optimization, not something that's going to speed up a fat Wordpress site.

It's an easy change so might as well do it if you can but I doubt it will make a measurable difference. Caching pages with nginx / varnish will probably be your biggest speed improvement.