This issue was encountered in Ubuntu 20.04 LTS, using GNOME.

Opening Opera takes 30-60 seconds, during which time the system gives no indication that it is attempting to open the application. After being opened, it can be closed and opened again reasonably quickly.

• There are error messages when opening Opera via the terminal:

  (opera:231402): Gtk-WARNING **: 14:10:24.091: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name
  (opera:231402): Gtk-WARNING **: 14:10:24.094: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name
  (opera:231402): Gtk-WARNING **: 14:10:24.094: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
  [231402:231402:0409/141108.748868:ERROR:CONSOLE(19)] "Uncaught (in promise) [object Object]", source: chrome://op-resources/modules/services/history.js (19)
  [231402:231402:0409/141108.748928:ERROR:CONSOLE(19)] "Uncaught (in promise) [object Object]", source: chrome://op-resources/modules/services/history.js (19)
  [231402:231814:0409/141114.437753:ERROR:udev_watcher.cc(97)] Failed to begin udev enumeration.

• If opera has been closed recently, it opens much more quickly. There are still errors in the terminal, but they are different:

  (opera:235537): Gtk-WARNING **: 14:30:46.047: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name
  (opera:235537): Gtk-WARNING **: 14:30:46.050: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name
  (opera:235537): Gtk-WARNING **: 14:30:46.051: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
  [235537:235638:0409/143054.462332:ERROR:udev_watcher.cc(97)] Failed to begin udev enumeration.

• I have noticed that many applications installed via the Ubuntu Application Manage (“snap?”) develop peculiar behaviors, which are fixed by uninstalling them, and reinstalling them via get-apt install commands in the terminal. I will attempt this soon.
• I uninstalled Opera and attempted to reinstall it with the apt package manager. There is no apt package for Opera, so I reinstalled it with snap via command line. The problem persists.
• Based on the advice here, I attempted to launch Opera using opera –no-sandbox. This didn't work, but I got a different set of error messages:

  (opera:7904): Gtk-WARNING **: 15:24:25.130: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name
  
  (opera:7904): Gtk-WARNING **: 15:24:25.133: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name
  
  (opera:7904): Gtk-WARNING **: 15:24:25.133: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
  [7904:7904:0410/152453.809696:ERROR:named_platform_channel_posix.cc(145)] connect /home/beloch/snap/opera/118/.config/opera/oauc_pipe_r: Connection refused (111)
  [7904:7904:0410/152503.812667:ERROR:named_platform_channel_posix.cc(145)] connect /home/beloch/snap/opera/118/.config/opera/oauc_pipe_r: Connection refused (111)
  [7904:7904:0410/152504.814020:ERROR:named_platform_channel_posix.cc(145)] connect /home/beloch/snap/opera/118/.config/opera/oauc_pipe_r: Connection refused (111)

• Based on the AskUbuntu thread "Very slow startup of some browsers,", I investigated appmenu-gtk2-module & appmenu-gtk3-module. Both are fairly small and seem to modify the system very little, so I installed them. This did not fix my issue. I also ran lspci -vnn | grep “VGA” in hopes I might see some relevant error, but the output seems benign to me.

  0a:00.0 VGA compatible controller [0300]: NVIDIA Corporation TU116 [GeForce GTX 1660] [10de:2184] (rev a1) (prog-if 00 [VGA controller])

• After installing GTK I foolishly did not open Opera using the command line, so I was not able to inspect any error messages that may have appeared during its slow start. However, opening Opera again produces two “sets” of errors, with the second being displayed after a fresh input prompt appears. This is new behavior. The second error message is also new. (It is very disorienting that my problem is consistent, but my error messages are not!)

  beloch@workhorse:~$ opera
  
  (opera:68236): Gtk-WARNING **: 16:02:08.016: Theme parsing error: gtk.css:1555:23: 'font-feature-settings' is not a valid property name
  
  (opera:68236): Gtk-WARNING **: 16:02:08.019: Theme parsing error: gtk.css:3591:25: 'font-feature-settings' is not a valid property name
  
  (opera:68236): Gtk-WARNING **: 16:02:08.019: Theme parsing error: gtk.css:4053:23: 'font-feature-settings' is not a valid property name
  beloch@workhorse:~$ [68337:68337:0413/160208.083377:ERROR:broker_posix.cc(43)] Invalid node channel message
  [68335:68335:0100/000000.179966:ERROR:broker_posix.cc(43)] Invalid node channel message

• Inputting lsmod | egrep 'nouveau|nvidia' in the terminal outputs:

  nvidia_uvm           1011712  0
  nvidia_drm             49152  4
  nvidia_modeset       1183744  8 nvidia_drm
  nvidia              19746816  370 nvidia_uvm,nvidia_modeset
  drm_kms_helper        217088  1 nvidia_drm
  drm                   552960  7 drm_kms_helper,nvidia_drm
  i2c_nvidia_gpu         16384  0

As I postulated earlier, the issue was solved by getting rid of SNAP. A friend was able to hold my hand through the process.

1. I started by uninstalling snap's Opera with sudo snap remove opera in the terminal.
2. On the Opera website I downloaded their .deb install file. (currently opera-stable_75.0.3969.171_amd64.deb, but presumably this is updated regularly.)
3. In the terminal, I installed this. I am told it is essential to indicate the full file path for this process (no ~/). Additionally, the 'tab' key was helpful in automatically filling out the specific filename. sudo apt install /home/beloch/Downloads/opera-stable_75.0.3969.171_amd64.deb

This completely resolved the issue. Opera now opens immediately, without any errors at all being displayed in the command line.