I have been looking into related log performance issues, and I noticed that large logs were very slow in Chrome. In particular, Chrome would hang when scrolling, and show just a blank page until it became responsive again, and selecting text was very delayed. Firefox did not have the same issue. I think the issue was at least partly caused by this bug in Chromium, which was just marked as fixed yesterday. I was able to verify that updating Chrome to a version with the fix for that issue (79.0.3945.130) caused the issues I was experiencing to go away, and for performance to be much closer to Firefox.
Are other users seeing this problem on Firefox, or only in Chrome/Chromium?
They are definitely some other things that should be improved, I just wanted to note that at least some of the problem seems to have been a bug in Chromium that was very recently fixed.
I think it is because the javascript function which loops each line and appended parallel label
https://github.com/jenkinsci/workflow-job-plugin/blob/master/src/main/resources/org/jenkinsci/plugins/workflow/job/console/NewNodeConsoleNote/script.js#L21-L42
It can be reproduced by a generating lots of lines in parallel step, such as
The consoleFull is very slow and firefox complaining about the js
jglick does it make sense to generate the label in backend(Jenkins side) ?