If you have a somewhat recent computer (Core 2 Duo or better?) it was probably caused by one of your plugins. It's pretty easy for a plugin to block the UI in ST2.
If you listen for on_modified as a plugin and do any work, the next keystroke won't appear until you return from on_modified. Multiply that across many plugins, some of which don't care about speed, and your editor becomes very slow.
If you listen for on_modified as a plugin and do any work, the next keystroke won't appear until you return from on_modified. Multiply that across many plugins, some of which don't care about speed, and your editor becomes very slow.