Read message appearing at the bottom of window.

Since installing Firefox 47 I've noticed that, in addition to the usual "Transferring data from..." and "Waiting for..." messages that I'm used to seeing at the bottom of a Firefox window, I am now seeing "Read..." messages at the bottom of the window, which I don't recall seeing in previous versions of Firefox. I've also noticed a corresponding slow down in my web browsing speed since the "Read..." messages started appearing. My question is, what do the "Read..." messages mean and could they be connected to the web browsing slowdown I've noticed? I've tried running Firefox is in safe mode, but doing this seems to provide only a temporary fix for the speed issues.