Why is Mozilla Thunderbird SO SLOW?

Back on September 23rd, 2002, Mozilla Thunderbird was created. Memory tells me that this email software was a faster, more efficient email option (when compared to Microsoft’s Outlook).

Over the years, it seems like Thunderbird became less and less efficient. In fact, and at least for me, it’s so slow and bloated that I often opt to just grind out lengthy email replies on my smartphone’s touchscreen.

Why is Thunderbird so slow? How to speed up Thunderbird?

If you can help me with this, please leave a comment.