No, The default themes are not the ones that are technically quickest. You can find many themes which are more quicker than the default themes. (It does not mean, the defaults are slower, they are good for default and branding. I like them also)
does running them slow down performance in any way, even slightly?
It depends, some themes may be faster than the defaults, some may perform slower. A rule of thumb is, the more the gorgeous, the more they use resource (hence slower).
You can find many themes here.
I found this PPA to be very good for themes. It is webupd8 themes PPA. To add and use the PPA, use these commands.
sudo apt-add-repository ppa:webupd8team/themes
sudo apt-get update
sudo apt-get install zukitwo-theme
Also you can install other themes then zukitwo. You must install
myunity software to change themes.
sudo apt-get install myunity
sudo apt-get install ubuntu-tweak