When you type quickly, the letter popups sometimes do not show up at all, or they are only visible for a few milliseconds so that you can hardly see the letter you just typed.

I used an HTC phone for some time, and the HTC keyboard had a nice feature: It also had letter popups, but those popups always stayed for a minimum time. Longer than you are actually pressing the button, but still not too long to be a distraction.

That way you could type VERY quickly and still verify each and every letter you hit.
With Swiftkey, I can also type very fast, but I sometimes dont know what I typed, because the letter popups are barely visible. This is especially annoying when typing letter-based, and not word-based (for example passwords).

It would be great if there was a setting for this.

(If I remember correctly, the letter popups on the HTC keyboard always appeared centered above the keyboard, and not above the actual key you pressed. That was also kinda nice, and easy to follow.)