Optimizing CSS performance
In short, don’t worry too much about your selectors. Unless you’re rendering 1000 items with box-shadows, gradients, animations and annoying hover effects they are all fast enough - including the * selector. What you should worry about is janky performance when scrolling, heavy CSS transforms and animations. Box-shadows and filters.
That being said, what you really should be spending your time optimizing is not the CSS but your heavy images, too many file requests which could be concatenate, your server setup and database queries etc.
Selectors are read right to left
Yes. Writing #Batman a will first find all a elements and then filter for those that are a descendant of an id called Batman. It would be better to write a simpler selector as a.Batman-link or even better .Batman-link.