I am looking at a few SEO tools for WordPress, WordPress SEO by Yoas and All in one SEO. A couple of plugins to help with the SEO of your website.
For performance I am comparing / contrasting 2 of the top plugins here as well. WP Super cache and W3 Total cache. Here is a quick comparison of the two plugins from this site.
I will be installing each of these items on a few websites and watching the performance over the next few days / weeks. Stay tuned for more information!