OS: Linux Mint 17 (aka 14.04)
CPU: Intel(R) Core(TM)2 Duo CPU T7250 @ 2.00GHz
Videocard: Nvidia 8400M GT (driver: Nouveau)
App version: 2.2.0 (KDE 4.13.3)
Screen resolution: 1440x900
Details: Maximized: kstars - 53-57%, X - 30-32% (if resizing the window to smaller one both CPU loads are reducing)

Question: Why? Is it related to GPU? If yes - how?

That's not KStars-bleeding, KStars bleeding is v2.3.0 and you can only get it on 14.10 and higher. At any rate, on my system, I'm getting 3% full screen, and when I stretched the KStars window to cover two full HD monitors, it jumped to a whooping 4%.

Edit: why aren't you using the NVidia properitary drivers?

For 14.04 it's bleeding version I don't use proprietary driver because I don't need it - 3D is working out of box with Nouveau (guys have progressed a lot since I last time checked them!) But also because under 14.04 proprietary drivers are killing the system for unknown reason Is KStars using any GPU functions?

OS: Linux Mint 17 (aka 14.04)
CPU: Intel(R) Core(TM) i5-3330 CPU @ 3.00GHz
Videocard: AMD Radeon 5500 (driver: Gallium)
App version: 2.2.0 (KDE 4.13.3)
Screen resolution: 1920x1080
Details: Maximized: kstars - 47-49%, X - 20-22% (if resizing the window to smaller one both CPU loads are reducing)

It does not seem right at all!

OS: Fedora 21
CPU: Intel Core i3 3110M @2,4GHZ
Video: Intel HD Graphics 4000
Screen Resolution: 1920x1080

App Version 1: 2.2.0 (KDE 4.14.3), default Fedora package
Details: Maximized, kstars needs about 11% CPU, X 5%

App Version 1: 2.3.0 (KF 5.5.0), bleeding edge from my repo
Details: Maximized, kstars needs about 9% CPU, X 5%

On my desktop with Radeon HD 5770 (gallium driver) and Core i7 2600K:
App Version 1: 2.2.0 (KDE 4.14.3), default Fedora package
Details: Maximized, kstars needs about 6% CPU, X 4%

App Version 1: 2.3.0 (KF 5.5.0), bleeding edge from my repo
Details: Maximized, kstars needs about 5% CPU, X 4%

So the values are fine, even when I use a very high star density and let the time run in steps of a minute I get less than 50% CPU usage.

Ok, how to explain my values? Everything by default

In KStars menu Settings you can switch between OpenGL and QPainter backend. QPainter puts a lot more stress on the CPU. Try OpenGL.

CS
Klaus

And that is a piece of advice! Thank you, klaus, it helped! But some strange graphic glitches appears

Well, when I switch to OpenGL, I get the message:

This version of KStars comes with new experimental OpenGL support. Our experience is that OpenGL works much faster on machines with hardware acceleration. Would you like to switch to OpenGL painting backends?

"Experimental" it is. But QPainter puts the laptop's fan always on and that is more annoying than the glitches.

CS
Klaus

Hate to be the bearer of bad news, but OpenGL will be disabled in future version of KStars since it is broken at the moment and need some work to get it working and we don't have anyone to work on that. If someone is willing to volunteer for this task, please let me know.

I received this message about the experimental status, so I understand that I need to ignore the glitches Yes, my laptop fan made me pay attention to the CPU consumption - i5 machine is a desktop machine which all the time makes noise regardless of the load
Sad news with OpenGL...
Offtopic - no luck with bringing the newest Ekos to 14.04? As I can understand I will have to use some black magic aka chroot-ing an 14.10 install