These strings are required for translating the validation messages you see when you submit invalid data in the contact form and in the ajax portfolio pages. Without them, you will have to manually edit the js/main.js file to translate these strings instead of using the po/mo files for translation mapping.
It should not affect SEO since it is part of a script and Google should just ignore the same.
For performance improvement tips, just have a look at this page for some help –
http://gtmetrix.com/wordpress-optimization-guide.html
Caching, combining and minifying the scripts, css should go a long way in speeding up your website by reducing server load, server requests.