Thanks for posting into these forums.
On twitter you mentioned that "key delay" is set to minimal, I'm not sure what you mean... how do I do that?
Is it possible to attach a sample document that exhibits the behavior? Or post a screencast showing the slow typing.
The performance issues that I'm aware of are:
In some fonts/languages there seems to be a bug in the underlying text system that can make things really slow. See http://support.hogbaysoftware.com/t/taskpaper-3-5-korean-text-performance/2304 I don't think this has shown up before in english, or alphabets that overlap english. To be on the same side make sure that you are using just the default stylesheet, there may be some cases that trigger this underlying bug brought on by custom stylesheets.
To my knowledge typing performance is good unless the problem in case (1) is in play.
All editing performance problems that I know about are related to the amount of the document that's displayed in the text buffer, not the total size of the document. So if you have a really big document and can break it into projects. It should be that you'll see a big performance improvement if you focus into a particular project before editing.