[asutherland]

There's several things you could be running into:

* Auto-compaction isn't working and so the .msf files are growing larger than they need to be. Thunderbird's per-folder metadata indices are stored in a weird texty append-only database format known as "mork" that you may have heard of, usually preceded or followed by some cursing. The file must be read completely into memory, and in many cases this ends up synchronously happening on the main thread. The upside to this is that the thread-pane is very fast and allows sorting on all columns, etc. Right-clicking on a folder and choosing "compact" will trigger compaction of the mork database as well as the offline cache (a per-folder mbox-style file where the downloaded messages are stored).

* Not leveraging Thunderbird's offline storage and proactive syncing of messages. If you bring up the account settings for the account, and look at the "Synchronization & Settings" tab, you want "Keep messages for this account on this computer" checked. You also want checkboxes by folders you care about in the list that pops up when you click the "Advanced..." button. With these enabled, Thunderbird will proactively download the message bodies to your computer, making them available for both offline access and rapid message display.

Note that if you only launch Thunderbird when you want to read your mail and immediately close it when done reading, you won't be getting much benefit from this and things may even go slower as you fight Thunderbird playing catch-up with your mail.

* You have a lot of of messages in the folders in question. The message file loading is O(messages) and so is sync without CONDSTORE enabled. (Thunderbird's CONDSTORE implementation regrettably has some bugs and was turned off recently. See https://bugzil.la/912216) Additionally, gmail's IMAP implementation can take some time to open an IMAP folder with a lot of messages because it needs to build a sequence number mapping and do other stuff.