Title says it all. Not sure if it's a browser issue (it's been happening on/off for months for me across a variety of Firefox versions), or possibly something on the server's end.
I'll just be using the quick-reply to make a post, and decide to double-check formatting/image embeds by hitting Go Advanced to get the post preview, and the page would sit there for 20+ seconds before proceeding, if it does at all. Meanwhile going through the forum and viewing other threads is fine and snappy during this time.