Story time.
I got a brand new computer and for some reason FR was slower than on my old computer that still has a sticker saying “Designed for Windows XP." Didn’t matter what browser I used, except Chrome was a bit less bad than Firefox. Pages would hang and sometimes freeze up for 10+ minutes.
I finally figured out it’s the Skype Click to Call plugin, which thinks various bunches of numbers comprise phone numbers that needs to be converted into a convenient button for you to click.
FR happens to have lots of these numbers in places such as: attaching items to PMs, viewing your lair (the page numbers), bestiary (more page numbers), hoard, etc.
Since the plugin apparently has these issues on other websites as well, I disabled it and the problem is fixed.
PSA in case it’s causing some of the complaints about FR being so slow on a new computer (probably not causing your coliseum complaints).