Version: 48.0.2564.116
OS: OSX

What steps will reproduce the problem?
1. Go to http://jsfiddle.net/greggman/TVA34/
2. Make sure Japanese input is on hiragana mode. Can be Apple's or Google's
3. Click in the top left area and type ABABABABABABABABABABABABABABABABABABABABABABABABABABA

What is the expected output? 

Input it smooth

What do you see instead?

Input lags up to 1 second per character. Even moving the cursor or pressing delete takes time. Type 50 random characters and you can frees the browser for many seconds.

Here's a video showing the issue

https://www.youtube.com/watch?v=eWSshQTqed8