I just recently started using Evernote and I installed the webclipper for Firefox, I have the latest 16.0.2. From the beginning it was very slow, click on the icon and wait 5-10 seconds for the dialog to appear to login or enter tags, then another 5-10 seconds after clicking on clip for it to give me the pop-up message at the bottom about editing it.
I lived with the slowness for a bit and made some clips but now after a few days it has started behaving even worse, causing a "A script on this page may be busy or it may have stopped responding ..." message. If I click continue, eventually the dialog box where you enter the tags goes away but no clip is made or saved.
Firefox Latest 16.0.2
Web Clipper latest 5.4
Evernote for the web, no local client installed
Windows XP Machine
More info, appears to only be completely unresponsive on some websites, continues to be slow on others. I can reliably reproduce the script error by going to the website below and trying to clip the article even after I re-start Firefox and that is the only page loaded.