Clang is from apple under a non-copy left license. Making all our development depend on it would be the most stupid move ever done ever.
All these apps (except video and audio) run noticeably slower to me in non-2D unity than in 2D unity. I use VLC for multimedia and it also feels faster in unity-2D.web browsing (Firefox)
shell sessions (local and remote via SSH)
office applications, primarily spreadsheets and documents (Open/LibreOffice)
Edit: Just read that you use Kubuntu. Well DUH, this is about unity being too slow.
Most of these things come from mainstream. Ubuntu mostly just packages stuff and does interface tweaks and develops unity. So I think it makes sense that in regards to what canonical plans to do next, it would be far more useful for them to opimize performance rather than do what they would instead do (That is, most likely add new lenses to unity so that it is able to do even more things that a web browser can already do).From reading postings here over the past couple of years the things I think require the most work are:
hybrid laptop graphics like Optimus
installations that don't run afoul of nomodeset
transparent interoperability with phones and iThings
convenient multi-language support
residual audio problems with pulse/Jack
support for various peripherals like webcams, fingerprint readers, etc.
We don't have many details of how they tested.While Ubuntu performance isn't on par with SUSE, Fedora, Arch, or Debian, it's still better than Windows... The Valve developers even admitted such.
"Ubuntu" itself is not bad for performance, I get great performance in games by running them from a single terminal without a desktop manager.
The issue is that the default desktop is too slow and comparatively slower than DEs that do the same sort of effects. (And this is a bad thing).
If Valve tested under a different DE than unity, then it does not say anything about whether we should improve unity or not.
(Actually, the main cultprit is not unity, it is compiz)