On the latest Tb 31.2.0, with HTML composing disabled, and no signature, I often experience this: I open a new/blank Compose window, or hit reply, and start typing in the main area, only to find that it appears Tb is not responding. 5-20 seconds later, every keypress or mouse click I had performed suddenly takes effect. After that, it may run smoothly the rest of the composition, or it may happen again once or twice, for an average 1-2 paragraph e-mail. I've experienced this on Windows Server 2003 and Linux Mint 17, both 32-bit editions.
I have no calendar plugins or anything composition-related installed that I can tell (only DivX Web Player, Google Talk, IcedTea, QuickTime, Shockwave Flash, VLC, WMP on Linux and there's the Messaging Menu and Unity Launcher integration 1.3.1 extension but no others; on Windows, no extensions whatsoever, and Foxit, MS DRM, Shockwave, Silverlight, MS WPF--all the defaults on both platforms). On Mint I'm accessing Gmail via IMAP and have a gigantic inbox, on Windows a private server via POP and my inbox has under a dozen e-mails in it (the rest are in archived folders according to year.) I have Global Search and Indexer disabled. On the Windows box, I have the OS-wide file indexing service disabled.
I have not tried safe mode yet, but I'm curious how video player plugins or a popup integration extension could interfere so detrimentally with a text composition window. I'm tempted to switch to a CLI for e-mail. The basics shouldn't be so painful...
UPDATE: In safe mode on Linux just now, I was manually disabling every plugin permanently, when I had a similar thing happen in the add-ons window, a 5s delay before the dropdown menu for "always activate/never activate" responded to my initial click.