I've been playing with the D3 as well as my Thunderbolt. Apps seemed to open slightly faster on the D3 and web pages (while on wifi) load about a second faster as well.
But the performance has honestly been pretty comparable with both phones. Then the shocker...I forgot that I had turned down my OC several days ago on my T-Bolt to 1.0GHz and I usually run it at 1.5GHz.
So after I turned my T-Bolt back up to 1.5GHz, I'm seeing the D3 is now a little slower to open apps, and a little slower to load web pages.
Point is this: I don't think the dual core processor speeds up the phone for normal uses (like opening apps and web pages). Sure, it's nice to have...but I'm seeing studders, home screen redraws, more than on my modded and OC'd T-Bolt running the same OS version (2.3.4) and HTC Sense 3.0 vs. this latest version of Blur (which I actually like...not love...like).
I'm not bashing the D3...I really do like it. But the dual core just doesn't put it in a different league like I thought it would. But maybe the 512MB of RAM is holding it back a little.