It seems to happen when:
1) Server is busy. The key strokes are being queued not processed.
2) A save in progress is under way.
You need very little bandwidth to send a key strokes. Even in the very old days you could type quite nicely over 300 bits per second (baud). That is thirty characters a second. Everybody has plenty of bandwidth these days.
What people normally have trouble with is down load width. When you are typing a post you using a little bit of upload and download bandwidth.
One easy way to test is to run the game software windowed with the link status indicator at the same time you are typing a post. It is a bandwidth or bad connection to Turbine your link in game will go down at the same time as your letters stop appearing in the forum post.
The difficulty is that there is nothing you can do on your end because it it a problem with Turbine equipment or a problem in the link between your home and Turbine.
The new improved forums that arrived middle of 2013 work lots better than the old forums.
Unless stated otherwise, all content in this post is My Personal Opinion.