gnome-shell growing use of CPU on 20.04

Question

My computer worked well until a few month ago. I began to notice some lagging and even freezing/lock. I checked with "top" and "htop" and notice a enormous use of CPU by the gnome-shell. I detected the problem well after my upgrade from 18.04 LTS to 20.04 LTS this winter and the beginning use of the NordVPN. It seems to worsening with time (yet to be confirmed).

Currently, "usr/bin/gnome-shell" uses a few % of 1 core of my CPU at startup. This use grows with time: after 24 hours, the peaks are well over 15%. After a couple of day, the use is at 10% min up to 35% and over at peak use, with no apparent relation with my use of the computer. At that moment, Ubuntu begin to lag and is prone to freeze.

Closing all programs (see below) doesn't change a bit of this excessive use of CPU. Neither does a logout/login. A reboot is the only solution. Now, I have to do it multiple times a week. At the reboot, I notice a warning about a user (the root account, not "root") who is still working on the computer.

I tried to use the Mate graphical interface: the result is different with the permanent use of 1 core at 100%. But doesn't seems to lag anymore.

"inxi -F" gives the following results:

System:
  Host: Isis Kernel: 5.4.0-73-generic x86_64 bits: 64 Desktop: Gnome 3.36.7 
  Distro: Ubuntu 20.04.2 LTS (Focal Fossa)
Machine:
  Type: Desktop System: PRIMINFO product: UNLOCK INSTALL v: N/A 
  serial: <superuser/root required> 
  Mobo: ASUSTeK model: P8B75-M LX v: Rev X.0x 
  serial: <superuser/root required> BIOS: American Megatrends v: 9903 
  date: 10/31/2012
CPU:
  Topology: Quad Core model: Intel Core i5-3470 bits: 64 type: MCP 
  L2 cache: 6144 KiB 
  Speed: 1847 MHz min/max: 1600/3600 MHz Core speeds (MHz): 1: 1652 2: 1645 
  3: 1694 4: 1652
Graphics:
  Device-1: Intel Xeon E3-1200 v2/3rd Gen Core processor Graphics 
  driver: i915 v: kernel 
  Display: x11 server: X.Org 1.20.9 driver: modesetting unloaded: fbdev,vesa 
  resolution: 1920x1080~60Hz 
  OpenGL: renderer: Mesa DRI Intel HD Graphics 2500 (IVB GT1) 
  v: 4.2 Mesa 20.2.6
Audio:
  Device-1: Intel 7 Series/C216 Family High Definition Audio 
  driver: snd_hda_intel 
  Sound Server: ALSA v: k5.4.0-73-generic
Network:
  Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet 
  driver: r8169 
  IF: eth0 state: up speed: 1000 Mbps duplex: full mac: 60:a4:4c:52:67:21 
  Device-2: Qualcomm Atheros AR9227 Wireless Network Adapter driver: ath9k 
  IF: wlan0 state: down mac: a0:f3:c1:ee:05:e3 
  IF-ID-1: tun0 state: unknown speed: 10 Mbps duplex: full mac: N/A 
  IF-ID-2: virbr0 state: down mac: 52:54:00:33:9e:48 
  IF-ID-3: virbr0-nic state: down mac: 52:54:00:33:9e:48
Drives:
  Local Storage: total: 25.93 TiB used: 15.35 TiB (59.2%) 
  ID-1: /dev/sda vendor: Seagate model: ST8000DM004-2CX188 size: 7.28 TiB 
  ID-2: /dev/sdb vendor: Samsung model: SSD 850 EVO 500GB size: 465.76 GiB 
  ID-3: /dev/sdc vendor: Western Digital model: WD80EFAX-68KNBN0 
  size: 7.28 TiB 
  ID-4: /dev/sdd vendor: Western Digital model: WD40EZRX-00SPEB0 
  size: 3.64 TiB 
  ID-5: /dev/sde vendor: Seagate model: ST8000DM004-2CX188 size: 7.28 TiB
Partition:
  ID-1: / size: 404.92 GiB used: 257.34 GiB (63.6%) fs: ext4 dev: /dev/sdb5 
  ID-2: swap-1 size: 7.68 GiB used: 0 KiB (0.0%) fs: swap dev: /dev/sdb6
Sensors:
  System Temperatures: cpu: 38.0 C mobo: 33.0 C 
  Fan Speeds (RPM): cpu: 1044 fan-1: 695
Info:
  Processes: 321 Uptime: 9h 48m Memory: 15.32 GiB used: 4.81 GiB (31.4%) 
  Shell: bash inxi: 3.0.38 

The Ubuntu updates are made every couple of days. I use Firefox, Thunderbird, Calibre, Transmission, Rythmbox, VLC and Oracle VirtualBox (VB) and the VPN of NordVPN. All stay open night and days. I use VB to run Windows XP and some flavors of Linux from time to time. I test Firefox Nightly every day (30') and Firefox Beta once in a while but no changes seem to appear while using them or after closing them.

I found Gnome Shell and high cpu usage but the user runs Ubuntu 18.04

Before introducing a bug, I'd like to known if someone has an idea on what to search for.

EDIT May, 25 2021: I needed a stable OS quick: I made a fresh install of an OS & app (Linux Mint-Mate + data migration) and the problem didn't appears anymore :-/

N.B.: So far, I kept the "buggy" config to make tests if suggestions appears. I'm not good enough to search deeper by myself but I do believe it's a config error somewhere, not a bug.