I'm not even going to speculate on the performance possibilities -- will Open webOS target now hardware, or hardware that was top of the line 6 months ago, or hardware that will be new 6 months from now? We don't know. Will the software overall perform better? We don't know. We all have this speculation in the community, that bringing a modern webKit into the equation will automatically make things better, that bringing a newer V8 into the equation will automatically make things better, but none of this has been proven, in fact, I think we've proven then it's a resounding "maybe".
webOS 3.0.4 brought with it a major upgrade in the Node V8 engine, and had some huge performance improvements, but it also had a lot of other adjustments particularly involving the use of dual core optimizations, that may or may not have covered all of that performance improvement. There's a brand spanking new QTwebKit browser in the files section here, that has no obvious improvements to speed of things.
SO, I think that a lot of performance issues that we have right now, are involving memory and a lot of places where there are very complex processes, even for things that seem quite simple. My favorite case for this, is that it takes my Pre3 approximately 4 hours to login to Skype, counting the time from hitting "Online" to having my entire buddy list synced to current statuses. There's a crapton of stuff going on, and it's definitely being done in somewhere real close to the least efficient way possible.
Hopefully, right from the get go, we will see design improvements that eliminate things like that. If not, we will need to make them. It needs to be done, because although we can continue throwing more and more hardware at things, the average mobile device still only has 512MB RAM, maybe 1024MB for the bigger stuff. I don't think I've heard of anything with 1.5G or 2G RAM (maybe the S3? someone told me it was "2 gig", but they weren't clear if they meant CPU speed or memory) . The other mobile operating systems basically spend all their processing time housekeeping for a single running user app, and whatever background services they perform, and are otherwise asleep. webOS never pauses or halts execution of a program (except, bizarrely, the web browser when you open a new window in the background) at any point, which leads us even more so into the lags and jitters, as well as the crushingly low battery lifetime that we get with our webOS devices.
We just don't know, at all, how big of a change Open webOS is going to be from webOS 2 and 3. We're mostly pretty sure that it will all be based on the same technology - V8 and WebKit with some sprinklings of QT and other possibly native codes. Will any of what we currently have in webOS 2 and 3 be there? I don't know. I hope that everything is getting a major revamp.