My experience shows it to be faster... but this could be nothing more then preception. It's really hard to determinem since I've never done a side by side comparison.
I found this response on the SE Newsgroup thread called ST6 Scorcard from Dan Staples back on 3/19/13
"You do not need to check the high quality rendering box for increased performance. Graphic performance was boosted by 2 or more times in ST6, independent of this checkbox.
What the checkbox does is let you get much cleaner highlights without increasing the triangle count. If you are working on a "swoopy" part and inspecting the way the light flows on it, it can look sort of muddy and you can see where the triangles are. If you hit this button, you will get great highlights without any performance penalty."
You may want to search the Newsgroup since this website wasn't around a year ago when ST6 first launched.