I THINK you and I MAY be seeing two different things. For me once Thunderbird is up and running, it is OK. It is just that it takes long enough to start that I am beginning to think I did not hit the icon properly.
I ran the updates this morning and no change. However, I did notice that my CPU usage held at 25% while it loaded. While I do not know how it was configured before, I am wondering if it went from being multithreaded down to just using one thread (core)? For me that would explain the increase in the time it takes to load and I doubt I push Thunderbird hard enough (while it is up and running) to notice the difference between using one core or all four.