Ubuntu 18.10 is much slower with Wayland than X.org

Question

I have just recently installed Ubuntu 18.10 on my desktop. There is an issue which I can't understand: if I start a session with Wayland (choosing this while logging in), the system noticeably slower. I mean, even the mouse and typing are with a small (but noticeable) delay. If I run htop, I see that all the cores are constantly 40-50% busy, and the most CPU-hungry processes are gnome-shell.

System info

Output of inxi -SCG (v. 3.0.24-00) under Wayland:

System:
  Host: mydesktop Kernel: 4.18.0-13-generic x86_64 bits: 64 
  Desktop: Gnome 3.30.1 Distro: Ubuntu 18.10 (Cosmic Cuttlefish) 
CPU:
  Topology: 6-Core model: Intel Core i5-9600K bits: 64 type: MCP 
  L2 cache: 9216 KiB 
  Speed: 1482 MHz min/max: 800/4600 MHz Core speeds (MHz): 1: 2315 2: 2317 
  3: 2318 4: 2318 5: 2320 6: 2320 
Graphics:
  Device-1: Intel driver: i915 v: kernel 
  Display: wayland server: X.Org 1.20.1 driver: i915 
  resolution: 3840x2160~30Hz 
  OpenGL: renderer: llvmpipe (LLVM 7.0 256 bits) v: 3.3 Mesa 18.2.2

The same under X11:

System:
  Host: mydesktop Kernel: 4.18.0-13-generic x86_64 bits: 64 
  Desktop: Gnome 3.30.1 Distro: Ubuntu 18.10 (Cosmic Cuttlefish) 
CPU:
  Topology: 6-Core model: Intel Core i5-9600K bits: 64 type: MCP 
  L2 cache: 9216 KiB 
  Speed: 801 MHz min/max: 800/4600 MHz Core speeds (MHz): 1: 800 2: 800 
  3: 800 4: 800 5: 800 6: 800 
Graphics:
  Device-1: Intel driver: i915 v: kernel 
  Display: x11 server: X.Org 1.20.1 driver: i915 resolution: 3840x2160~30Hz 
  OpenGL: renderer: llvmpipe (LLVM 7.0 256 bits) v: 3.3 Mesa 18.2.2 

I have a built-in Intel graphic card, the motherboard is ASRock Z390 Extreme4.

UPD

Under X11, there is another issue though. If I try to open the system settings, I am forcefully logged out. I created a separate question about this.

Disclaimer: I am not that advanced Ubuntu/Linux user.

Where can I look into the problem in my Ubuntu? What to start with?