Wow, I just gave it a spin, and the performance of this thing is ABYSMAL. It is not ready for beta testing. Period.
All preference windows (or basically any window besides the main browser window) start out completely blank, except for the titlebar and default button. You must then hunt around with your mouse, or start hitting the Tab key, until individual UI elements start getting drawn. If you can find a tab bar, that's best, since switching to another tab will redraw all the contents of the window.
Any mouse clicks take at least the better part of a second to take effect. Yeah, fastest browser indeed.
A much less serious issue is that the option to show page loading progress "in" the address bar wasn't what I expected. From how they worded it, it sounded like the URL field would "fill up", Safari 1-thru-3-style. Instead, it was simply replaced wholesale with two little progress bars with text labels beside them, then reappeared when the page finished loading. Should've said "in place of the address bar" or something along those lines.
Welp...looks like Safari still has no real competition on OS X unless you're willing to settle for Firefox. Kinda disappointing.