Why is applying formatting so slow?

Moments ago (c 9:30am Pacific 6/12) I entered the text of a reply (Dependency Hell) and then wanted to add code formatting around various words and phrases. So I highlighted the text and clicked the </> icon. Each time I did this there was an extended period, more than a minute I think, in which the cursor did not display and the interface was unresponsive. I’ve noticed this before, although usually not as extreme.

Using FF 60.7.0esr (32 bit) on Win 10 (64 bit). I’m connected remotely via VPN, and I have a lot of other FF windows/tabs open. I was able to use other windows in FF while this was happening.

I tried hiding the preview window, but it didn’t seem to help.
Ross