After launching Opera I see:

Your profile could not be opened correctly.
Some features may be unavailable. Please check that the profile exists and that you have permission to read and write its contents.

I have full read/write permissions to every file in my profile directory so message is bogus. I run Opera using command line and among messages I noticed:

[15114:15125:0609/192638.930581:ERROR:history_database.cc(50)] History failed to migrate from version 43. History will be disabled.

I moved History and History-journal files elsewhere and all starts up, though I've lost full browsing history as expected. Seems like upgrade corrupted my History and/or History-journal database. I checked them. It seems like they are SQLite3 databases. Is there any way to recover/fix those files?

@kened: I've spent several years building this profile... and actually this work/profile and my habit are the only things to keep me with this browser.

Stable release is supposed to work out-of-the-box... or at least it's my tendency with my programs.

Start & 10s & Freeze -> Restart & 10s & Freeze -> Restart & 10s & Freeze -> Restart & 10s & Freeze -> Restart & 10s & Freeze...

Completely unusable/broken. May I remind you this is supposed to be stable release... third or forth major stable release (69, 68, 67, ...) that's completely unusable before set of fix-builds and requires downgrade or some reconfiguration. If you require some configuration changes, either integrate it with installer (non-breaking/no-data-loss) or provide some steps...

@paulsg1: Same on Windows 10 - so bug is system independent and doesn't depends on my profile, therefore within core of the browser...

https://i.imgur.com/hYy4ojR.png
https://i.imgur.com/oV972me.png
https://i.imgur.com/gKfaeIe.png
https://i.imgur.com/XsFmjXG.png

First start works just fine, I check if workspaces work. Then I close Opera, start it again and after few seconds one core is working at ~100% (8 total therefore 12,5% in Task Manager), browser is not responding.

Downgrading Opera, I hope

Launcher.exe --check-for-update-interval=0

still works... if not, I'd be glad to

On the downside downgrade on Linux cleared quite a lot of my cookies... so another bug found.

It started not so bad first time (except for Opera GX advertisement on platform without Opera GX support) but after reopening it some time later it froze. Nothing helped so I was forced to kill it. On retry I noticed 95-100% CPU usage on initial process within 15-25s from start, browser basically freezes no reaction... basically same as https://forums.opera.com/post/200214 and https://forums.opera.com/post/200287 , possibly related to: https://forums.opera.com/post/200290 . Only way to "solve" this is to kill entire process tree. No warning... no error... no anything... nothing after restart as well. This basically renders browser in this state completely useless for me.

Ubuntu 18.04, my standard setup: ~200 tabs open (almost all hibernated because Opera eats tons of RAM nowadays), about 30-40 extensions and few thousands bookmarks. Nothing really special, same setup works on previous release and on some other browsers.

https://i.imgur.com/e4zMxyd.png
htop with part of opera process tree.
http://i.imgur.com/2OUlucq.png
"solution"

Downgrade solves the issue... therefore stable is unstable. Therefore:

sudo apt-mark hold opera-stable

opera-stable set on hold.
but I guess "security risks" and all the "mumbo jumbo" should prevent me from keeping this state for long.

I receive random lagspikes when... well, every now and then when doing basic stuff, for example: when opening YouTube video and scrolling, or when opening new tab or closing old one... it's freezing cursor in place. It seems like GPU processing related but I'm not sure. No issues on Chrome or Vivaldi so it must be Opera related.

I noticed some errors in dmesg:

traps: opera[28350] trap invalid opcode ip:558ba5db66bd sp:7ffe5fc51760 error:0 in opera[558ba333f000+6343000]

also some other data in logs:

ERROR: apport (pid 28687) Tue Oct  8 19:20:56 2019: executable: /usr/lib/x86_64-linux-gnu/opera/opera (command line "/usr/lib/x86_64-linux-gnu/opera/opera\ --type=renderer\ --field-trial-handle=11835000237524188079,3309384016044030391,131072\ --disable-features=SharedArrayBuffer\ --lang=en-US\ --enable-auto-reload\ --ab_tests=DNA-70598-ref:DNA-70598\ --num-raster-threads=4\ --enable-main-frame-before-activation\ --service-request-channel-token=10174150109811527058\ --renderer-client-id=170\ --no-v8-untrusted-code-mitigations\ --shared-files=v8_context_snapshot_data:100,v8_natives_data:101")

so I reverted back to latest 63 and problems are gone... so the post title is quite wrong
Linux 5.0.0-31-generic #33~18.04.1-Ubuntu SMP Tue Oct 1 10:20:39 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux with latest Nvidia drivers.