phew, this got quite tl;dr. Sorry. Kudos if you sit through!

I'm a KDE guy. I used KDE first when I first used Linux. Ever since I preferred KDE. Of course I had my stroll, yet I always come back to KDE.
So many features. But features produce bugs. (For example the current version crashes on me every second.) But this is a different topic, let's keep the bugs out of the discussion for now.

So. When I'm working on Windows 7, I have that... snappy and minimalist feel. Visually, and performance wise.
What do I mean by this:
- Alt-tab is instant. No animations, no silly effects. Efficient, easy, simple.
- Alt-tab provides a 4-direction window switching. (Even with Aero Basic)
- Even with no effects it, it looks OK. Of course "Classic" looks a bit retro now in 2013, but it's still OK. Aero Basic is totally perfect.

Now. In KDE the default Alt-tab switcher is really slow. Especially if you have a "bad" GPU. Like fglrx, ati, or even nvidia is slow.
You can't move 4 way either. It's not fast. It's there but quite terrible.

Of course it's KDE so you have many options. There are many other ones to use. Sadly, none of them offer the same snappy experience.
The effects are something else. Put away your KDE hat and check KDE with default effects, and then without any compositing. Notice it?

Now... you all say... "But this is open-source..."
Yes it is. I could... theoritically I could learn Qt, QML and KDE's code and the necessary C++ skills. Maybe it would take weeks, months, years to get it right. But very well I could.

Making an instant icon-only 4-way switcher is a few hours of work. One hour? Maybe?
Widget style and decorations: Something fast, "lightweight" (the feared buzzword, hah!), "solid".
"Skulpture" was a great example, until the artist/coder closed the shop.
If KDE could adopt that theme... we could tick both things.

And a plasma style which ... well, couldn't find one yet. The thing is... when you go to the styles in KDE, you see a LOAD of themes.
70% of these are dead links, 20% are not working... and the rest 10% will work somewhat.

So what do you guys think? I mean I know what I'm talking about, I'm working daily on Windows and Linux. But what is your opinion on things?

To sum things up:
I propose a set of new things to make KDE snappy, easier to use for fast work (power users).
Decoration: Skulpture (see above)
Widget: Skulpture (see above)
Task switcher: Needs to be written. Modifying one could work. See above.
Plasma style: Suggestions needed (working with 4.10 or so.)

What are your computer's specs and KWin version?

All switchers feel snappy enough here using KWin 4.9.5 on a Core 2 Duo laptop with Nvidia GT 240M (310.14 proprietary drivers) with/without compositing enabled. It was a midrange laptop 4 years ago.

You can also try the switchers available on GHNS or write your own using just QML - the "Windows Wall" I'm using atm seems pretty simple at 257 lines.

Update: Never mind the above, just noticed you already got some great suggestions at viewtopic.php?t=109935

I realized I messed up the post a little bit. Corrected the post.
Skulpture is not a plasma style, it's a widget/decoration engine.

Anyway. There are my suggestions.
As for the switcher... maybe I could make that work. Heck, I guess it's pretty easy (fingers crossed) to add two new bindings and make it INSTANT.
The others are not. How do you check if something is obsolete, how do you maintain things?
Mainly talking about Skulpture and a plasma theme.
Is there a tool to test if something is obsolete in them or needs to be fixed? (Like GTK throws you 500000 errors when you use any GTK apps. This deprecated, that's wrong, this is obsolete, that's failed. Aaalright, it's GTK, it's expected.)

To answer your reply:
I tried KDE 4.9.5 and KDE 4.10.1 on the following computers:
- Intel i5 2500K with Radeon 7870OC -> laggy, slow, buggy (ati and fglrx both)
- Intel i5 2500K with GTX 660Ti -> laggy, slow, buggy (nouveau and Nvidia both)
- Intel C2D P8600 with Radeon 4xxx Mobility -> laggy, slow, buggy (ati)
- Intel C2D P8600 with Intel -> snappy, fast

It's kinda sad, I know. With Intel GPU KDE works really good. With others, not much.
I tried disabling VSYNC and other did fiddle with other options, but they didn't seem to do much good.

I think I've started getting the gist of what you're saying.

1. KWin's window switching implementation feels slow, compared to that of Windows
2. It does not support 4-way switching
3. The Oxygen theme and window decoration feel slow
4. The Skulpture theme and window decoration felt snappy
5. ...but they haven't been updated to work well with recent KDE SC versions
6. There is no matching Plasma theme for Skulpture
7. There is too much API churn in KDE.
8. The desktop feels snappy with an Intel GPU.
9. It doesn't feel that snappy with AMD/Nvidia GPUs
10. ...and even exhibits buggy behaviour

Here are my comments on these:
1. As indicated by KWin's developers, this can be because of a specific switcher's animation speed, and also because of the standard delay for "switcher-less" window switching. The former you'll probably solve in your window switcher. KWin's developers will add a knob for the latter, IIRC.
2. No idea what this is; I'll check it out when your switcher comes out, again.
3. Agreed, it is highly dependent on its settings and the application used however
4. No experience with these
5. That's a shame, can't really help you there though. You could try to reimplement them with some other engine I guess, perhaps QtCurve?
6. No big loss I guess, since most Plasma widgets are designed for the Oxygen plasma theme and look fugly with most other themes anyway
7. I'm just a Python code monkey, can't really help you with that. However, I think you're confusing the introduction of new APIs and functionality with breaking backwards compatibility, which IIRC is no go in all but very exceptional circumstances (Nepomuk 2?) across the KDE 4.x series
8. It's been a long while since I last suffered an Intel GPU but, from what I hear, I think you got lucky with the version of your Intel drivers
9. ...and got unlucky with the drivers for these, or the hardware combos
10. The combo I've had the most success with during the last few years has been Nvidia cards with the proprietary drivers, especially since they decided to implement XRANDR. As they say, they're the worst, except for all the others.

I'd suggest you keep your patience and wait for Wayland. Apart from being a saner choice for a display system, the mere process of porting applications to it exposes a myriad of performance bottlenecks in applications and drivers, which will hopefully get fixed in time. Witness the amazing progress in KWin for example, benefiting even X11 users.
Another big boost to snappiness will come as KDE applications begin to take advantage of Qt Quick 2.

There are also other factors, such as code quality, hardware performance and kernel scheduler behaviour. Let's not get there however!

P.S. Please don't make such extensive changes to your posts after someone has replied to them. Not only did I have to reread your first post, but it also makes my second post look out of place, driving potential participants out of the conversation.

I'm just a Python code monkey, can't really help you with that. However, I think you're confusing the introduction of new APIs and functionality with breaking backwards compatibility, which IIRC is no go in all but very exceptional circumstances (Nepomuk 2?) across the KDE 4.x series

To clarify this issue: KDE guarantees binary compatibility (BC) for the development platform (kdelibs) for a whole major version, which means that API can be deprecated or added, but not removed.

Also now kdelibs is frozen for any feature commit, pending work on the KDE Frameworks 5 (KF5), so existing stuff like Nepomuk2 were born first to bring changes without breaking existing applications (which still use the old API) and also in line with KF5's goal of modularizing libraries.