User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0

Steps to reproduce:

Click the tweet button and start typing into the text box. Sometimes it'll take a full second between each keystroke. I type into the URL bar and copy/paste into the textbox instead.

Actual results:

Typing is very slow

Expected results:

It should not have been

So this is the UI that I get if I pick "Try the new Twitter" in the menu that comes up when I click on my profile image, right?

I just tried it (in both nightly and Firefox 65), and typing doesn't seem particularly slow... Just to check, do you see the problem in a clean Firefox profile?

(In reply to Boris Zbarsky [:bzbarsky, bz on IRC] from comment #1)

So this is the UI that I get if I pick "Try the new Twitter" in the menu that comes up when I click on my profile image, right?

Yup. Which thanks to you, I figured out how to disable. I don't recall turning it on.

It's intermittent/doesn't do it every time. No idea what triggers it, or what makes it go away. But hopefully switching back to legacy solves the problem. I reported the problem to Twitter in the survey I got when I switched to legacy mode. So they can deal with it, since they caused it.

The new layout is now forced, so the bug has returned.

Daniel, do you have time to take a look?

I can't reproduce (and I verified I'm using "new twitter"). I tried loading my timeline, hitting "pagedown" a bunch to populate a backlog of tweets, and then clicking "tweet" and mashing the keyboard as fast as I can. I don't notice any delays between keypresses.

Techni Myoko: have you noticed any particular actions that you can take to make the delay more likely to reproduce? Any tips on how to make it happen would be appreciated.

In any case, I try to take a closer look sometime this week and see if I can figure out a way to reproduce.

It doesn't take anything to reproduce on my end. It happened almost every time I tried to tweet

However, there's a plugin called "good twitter", that forces the old twitter style. And the problem is semi-solved till twitter forces it again.

Is there any chance you could record a performance profile while you try to type on twitter? See https://profiler.firefox.com/docs/#/./videos-intro for an example. That'd give us an idea of where time is spent on your machine.

Installed the plugin, will try to capture it

Took a while but I caught it

https://profiler.firefox.com/public/93595ee0a3733c89331d80e03dd684508fb22da9/calltree/?globalTrackOrder=0-1-2-3&localTrackOrderByPid=5156-1-0~11156-0~8456-0~&thread=2&v=4

Thanks, Techni (and sorry for the delayed response).

That performance profile is really useful for classifying this -- it shows that we're spending all of our time executing JavaScript, not doing layout. Here's the relevant piece of the performance profile, during the janky typing:
https://perfht.ml/34V0AVt

I'm reclassifying this to the JavaScript Engine for those folks to take a look at. (I'm also clearing the "Version" flag, to avoid misunderstandings -- this was filed for version 65, but the profile is using version 69, and there's no reason to think that the issue has gone away as of yet.)

Resolving this as incomplete; but if this is still occurring please re-open with a new profile and we will take a look.