(SeaMonkey 2)
For a while now I've notice that the right-click context menu was slow to appear & heavy on system resources (CPU load).
So I open a second Profile & note that it doesn't seem to have the same issue. I figure it is an extension that is causing the problem. So one by one I go about adding the same extensions as in my main Profile into the second, testing after each extension install. And as you would figure, I get the last extension installed & I still don't have a problem.
So I figure I'll remove all the extensions from my main Profile & reinstall them & see what happens. I was going to remove them all at once, but then I figured I may as well remove them one at a time. And I'm going to do them in alphabetical order. So the first was Adblock Plus. Uninstall. Restart. Test - & it's fine!? So I reinstall ABP. And it's still fine

Strange goings on.