Terrible dual monitor desktop performance with Intel Iris Xe on Ubuntu 20.04

Using the standard desktop environment, with or without Wayland, the performance is terrible when using two monitors. Interestingly, watching 4k@24hz video on the second monitor is no problem at all, but GNOME is really laggy. But simple things like scrolling in a GTK window, or moving it, lags intolerably. It works fine on Windows.

I'm not sure what information would be useful to troubleshoot this, so please just ask. I'm on Mesa 22, but the older versions I tested were not better. Here's a bit of glxinfo.

    Vendor: Intel (0x8086)
    Device: Mesa Intel(R) Xe Graphics (TGL GT2) (0x9a49)
    Version: 22.0.0
    Accelerated: yes
    Video memory: 3072MB
    Unified memory: yes
    Preferred profile: core (0x1)
    Max core profile version: 4.6
    Max compat profile version: 4.6
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.2

This is my 20-intel.conf:

Section "Device"
    Identifier "Intel Graphics"
    Driver "i915"
    Option "TearFree"    "true"
EndSection

I added this to make mpv play tear free videos. (This is also why I upgraded Mesa, and run Wayland.) Removing it has no impact on the desktop performance.

Some other info:

System:
  Kernel: 5.11.0-46-generic x86_64 
  bits: 64
  Desktop: Gnome 3.36.9 Distro: Ubuntu 20.04.3 LTS (Focal Fossa) 
Machine:
  Type: Laptop System: LENOVO product: 20VD v: ThinkBook 14 G2 ITL 
  Mobo: LENOVO model: LNVNB161216 v: SDK0J40697 WIN 
  UEFI: LENOVO v: F8CN34WW(V1.12) 
  date: 02/01/2021 
CPU:
  Topology: Quad Core model: 11th Gen Intel Core i7-1165G7 bits: 64 
  type: MT MCP L2 cache: 12.0 MiB 
  Speed: 1450 MHz min/max: 400/4700 MHz Core speeds (MHz): 1: 3504 2: 3388 
  3: 4100 4: 3698 5: 3835 6: 4023 7: 3916 8: 3347 
Graphics:
  Device-1: Intel driver: i915 v: kernel 
  Display: wayland server: X.Org 1.20.13 driver: i915 
  resolution: 1920x1080~24Hz, 1920x1080~60Hz 
  OpenGL: renderer: Mesa Intel Xe Graphics (TGL GT2) 
  v: 4.6 Mesa 22.0.0-devel (git-b5b105d 2022-01-21 focal-oibaf-ppa) 

Edit: There are versions of this ThinkBook that also includes an Nvidia adapter, but I have one that only comes with Intel. Per lspci:

0000:00:02.0 VGA compatible controller: Intel Corporation Device 9a49 (rev 01)

I don't think that you'll be happy with Ubuntu 20.04 and its rather old kernels. Running a T410 (I think your hardware is really cool!) I don't have any issues with the old kernels - but that device is old.

I'd suggest you try to install a new Ubuntu (21.10) like described here.

The newer kernels should support your far newer hardware. You should be aware that those versions are short lived, 22.04 will be the next LTS