Extreme slowdown on typing moderate amounts of text

[tonydroy]

The Rich Text area becomes very slow and laggy after typing just moderate amounts of text. Interestingly, cutting and then pasting back the exact same text removes the slowdown. I have attached a minimal file to exhibit the issue.

1. Start the program -- it doesn't do anything but open a blank RTA window.
2. Type about 15 lines of text (random typing or whatever): by the end, the RTA becomes very slow.
3. Cut and then paste back the exact same text: Now typing can continue without the lag.

Of course, this might be somewhat different on different setups. I'm running on a reasonably recent Windows machine.

SlowdownTest.zip