I don't really understand what you mean here sorry. What's the operations that have been known for a long time to be sub-optimal? And what exactly is "that" in the context of "To me that sounds orthogonal to the notion of freeing memory ranges..."?

Arc is actually primarily a Copy-On-Write system. You can put big chunks of data in it and share it around almost for free, but once shared, you can't modify it.
If you can prove that nobody else is going to be modifying the data, then you can convince the compiler to let you mutate it while shared.
For example: Arc<Mutex<String>>, where the mutex acts as proof that nobody else will be modifying the String.

I agree. Qt Creator starts really fast, and lets you navigate C++ code quickly with Ctrl+Click/F2 to go to the definition, F4 to switch between C/H files, Alt+Left/Right to go to previous cursor locations, inter-file refactoring of names, and when there is stuff on the screen you don't want, you mash Esc until it all goes away.

Settings -> Configure Toolbars...

Most KDE software is very, very flexible.

No, it wont. Everything in KDE is optional, even Systemd. Unfortunately if you want to use Kwin on Wayland it will require Systemd, but that is not the fault of KDE developers.

http://gcc.gnu.org/ml/gcc/2005-01/msg00008.html
There you go. Purposely making worse software is not how you make friends.

KILL THEM WITH FIRE O.O