On what version of Firefox you are observing the slowdown?

Yes, the web-ext API is pretty limited in terms of modifying the UI . This is what's supported at the moment: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/user_interface

You may try the new version - 0.3. It fixes some cases when the progress bar was not displayed. I don't think the failure to display it is related to intranet/internet - for the browser (and this extension) they are all the same.

Edit: v0.4 add option to place it at the bottom.