That 10% performance improvement estimate appears to be very close to the mark, at least on my MacBook Pro. Using 0.9.3, Handbrake encodes would average around 27 - 29 frames per second when transcoding a VIDEO_TS folder to H.264. In 0.9.4, using the same settings, I'm seeing encoding rates of closer to 30 - 34 frames per second. This means that on my Mac, Handbrake is now transcoding DVDs in real time or faster thanks to the improvements in the new version.
The Handbrake team has trimmed some of the fat from this release. There are no more presets for the PSP, PS3, or Xbox 360; all three have been replaced with a "Normal" preset that should work on any device that supports Main Profile H.264. Handbrake has also removed support for transcoding to AVI, OGG, and XviD. Personally I won't miss any of these, as I always transcoded to H.264, but for people who are still clinging to AVI (for whatever reason), this update will force them to abandon the format.
Another new feature that looks like it'll be incredibly useful is Live Preview, which allows you to encode a small portion of the video source using current settings and then see what it looks like. This will keep you from having to encode the entire video only to find out two hours later that one of your settings was off, forcing you to start all over again.
Handbrake 0.9.4 is a free download, available here.