Excessively slow script(s)
Currently, I get 2 warnings per page loaded from translatewiki.net, before I can do anything, such as scroll it, or type translations:
A significant amount of time passes between these occurrences, and I currently do not risk to stop the script, since I do not know which harm that may cause.
Typing translatons is often grossly delayed, too. As it appears to me, a script is running periodically every few seconds and while it is running, typed keys are not echoed to the screen. They appear later, then. This can make up for roughly up to 50 keystrokes when typing fast.
These effects together make typing an unleasant experience, and they consume a lot of real time.
It could be anything in the new jQuery release https://gerrit.wikimedia.org/r/#/c/16864/
Looks like you had already found the problem, but I tried to do the profiling you requested... http://toolserver.org/~nemobis/TWN-JS-firebug.tar.gz When Siebrand disabled ULS it went down from 18 s to 1 s (not related to LQT or anything else I could test). Firefox 14.0.1, fedora 17, AMD E350.
Works for me. More information is needed (like your computer specifications, operating system, browser and version). Please disable all your browser plugins while testing.