I forgot about this problem after I posted a video about it a while back. Just going by my totally unscientific test on an old laptop I'm using at the moment, it seems related to the max length of the longest string in the current code view.
I created a new project, created a window.open and held down the 'a' key and lifted the key when the line actually rendered and I got the following output:
I dont know if that helps steer you in a direction, but the delay seems "width" related not content related. I blanked that code and crated the line:
I pasted that in about 1000 lines (with LF at the end) creating a huge code block of a's then I held down a on a fresh line and it started rendering after it got to the end of the a's above it.
The above was done in 2017v1.1, I'll go and put the latest beta on here and see what difference it makes (if any).
Ok, just put 2017v2.1 on and followed the same steps as above (new project, window.open, hold down a) and no text appears until I let go of the a key.
So its a definite change between the versions. The latest version seem to act like like above would act with a string the width of the visible page (if you know what I mean).
When holding down a on a blank coding page, it takes a while (obviously) but the scrollbar starts widening as if its putting the a's in, but the a's don't actually appear until I let go of the a key.
It might be useful to test it on a bare metal windows install (boot camp into windows) rather than a VM (if you havent already) as it might act differently.
My tests didnt show a delay as long as Mathias though, but there could be lots of reasons for the difference if there's an underlying issue.