Many compliments have been received on how fast thunderbird is, so it's difficult to answer your question. I am not a network guru, but from my own experience, such delays are usually with the individual's internet connections and server loads. That is, the overall issue of speed involves many variables, only one of which is the email client. Today, for example, My Gmail account is slow in sending, but it is normally fast.

I have an ATT broadband connection which today is zooming along at 900+ Mbps both up and down. Very fast, yet my T-bird is so slow. It takes so long to parse my inbox. Is there a T-bird setting somewhere I can flip, to increase efficiency? It was fine for a week or so, but now it's back to tortoise speed.

Sorry, I am not that technically knowledgeable on internet connections. But someone else here may have suggestions. My only other thought is that, despite internet speed, servers may be overloaded at times or undergoing updates. That has happened to me, but may not be relevant to your situation,