gnome-shell eats the entire CPU (99% usage).

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I've got the very same problem on my Lenovo Yoga 2 Pro (which has quite OK hardware). This means I am also stuck on the 4.2.3-300.fc23.x86_64 kernel, which fortunately doesn't make gnome-shell sluggish. For the record, not even an external screen on 4.2.3-300.fc23.x86_64 gives me sluggishness.

We're both on Intel hardware tough, but mine is slightly newer:

~$ lspci 
00:00.0 Host bridge: Intel Corporation Haswell-ULT DRAM Controller (rev 09)
00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 09)
00:03.0 Audio device: Intel Corporation Haswell-ULT HD Audio Controller (rev 09)
00:04.0 Signal processing controller: Intel Corporation Device 0a03 (rev 09)
00:14.0 USB controller: Intel Corporation 8 Series USB xHCI HC (rev 04)
00:16.0 Communication controller: Intel Corporation 8 Series HECI #0 (rev 04)
00:1b.0 Audio device: Intel Corporation 8 Series HD Audio Controller (rev 04)
00:1c.0 PCI bridge: Intel Corporation 8 Series PCI Express Root Port 1 (rev e4)
00:1d.0 USB controller: Intel Corporation 8 Series USB EHCI #1 (rev 04)
00:1f.0 ISA bridge: Intel Corporation 8 Series LPC Controller (rev 04)
00:1f.2 SATA controller: Intel Corporation 8 Series SATA Controller 1 [AHCI mode] (rev 04)
00:1f.3 SMBus: Intel Corporation 8 Series SMBus Controller (rev 04)
00:1f.6 Signal processing controller: Intel Corporation 8 Series Thermal (rev 04)
01:00.0 Network controller: Intel Corporation Wireless 7260 (rev bb)

So maybe somewhere in Intel's land of the kernel space we've got a degrade in recent kernels?

1. Try a different laptop to rule out a bad video card.
2. Try "GNOME Classic" as Aeyoun suggested to rule out possible problems with shell extensions.
3. Take a look at the output of dmesg and/or "journalctl | tail " to see if there are any errors related to the video driver at the end.

Unfortunately, in my experience Gnome works better on newer hardware that what you have. Just running the plain desktop with no apps will require 1 GB of RAM. This is true of all 3 laptops I have. I suggest at least 4 GB of RAM and a machine capable of 64-bit (to be new enough and fast enough) if you want to have a good experience with Gnome.

That's not to say that you can't get it to work. But due to the age of the machine, it's less likely. I' m guessing that you're running into a video driver defect that's triggering bad behavior in the Gnome Shell or a defect in Gnome Shell that is only triggered by your video card. Filing a bug report against the video driver and/or Gnome Shell would be helpful.

For the age of the machine you're working with, I would echo Florian's recommendation to use Xfce instead. Gnome just isn't a good fit for your hardware. But, Xfce functions well on older machines. It can also be configured to look like Windows or Gnome if you dig into the preferences of all the GUI panels. LXDE is an option also, but personally I find the functionality and look to be too basic. Most people will be happier with Xfce I think.