I'm pretty confident that's exactly what's happening: Outlook cannot keep up. It is so busy hogging resources to do things like change all your hotkeys and randomly archive emails, open tab metaphors, display animations, run predictive text services, etc., that, at points, there is no memory left over to keep up with a reasonably fast typist. Multiple times, I've seen my email compose "drip drip drip," well behind my typing, and often at these times, I'm overflowing Outlook's buffer, so it begins to drop characters.
This goes back to what I said before: Microsoft keeps trying to stuff an overcomplicated, over-featured application, similar to what you'd install on your desktop, into your browser. So Outlook behaves erratically and unstably, wastes resources, and frustrates users with delays and errors. As astonishing as it is, Microsoft simply refuses to learn how to write a clean, efficient, helpful browser-based email client. But Microsoft has often been bad at managing user experience, and has a history of overstuffing and overcomplicating what would be better served by pared-down elegance. There's a reason Zune wasn't an iPod killer and Microsoft Phone wasn't an iPhone killer. It's the same reason Outlook is a sub-par email client.