Site performance optimization

I'm trying to improve performance on my website. I used a performance analysis site and it says I should move static content to a cookieless domain. What do cookies have to do with performance?

And any other tips for increasing the speed of a website? I'm thinking I should do some sort of in-memory caching, like Redis or memcached or something? But my hosting has pretty limited RAM.

Should I just go with some free AWS tier for like Elasticache or some shit instead of having everything on-disk on my main hosting?

>>12
the CDN I'm using does caching and minifying though

imagine getting mad at someone for using a caching solution