UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.63 Safari/537.36

Steps to reproduce the problem:
1. Open gmail
2. Send a long text message to yourself. My specific message is 1439 lines long and 114017 bytes. This is not an attachment, but the message itself. I can not attach it since it contains data from another person which might be private. But I don't think it contains any special characters.
3. Reply the message.
4. Try to write something.

What is the expected behavior?
Being able to write at least 2-3 lines of text in the top of the message.

What went wrong?
The first 5 letters I write pops up quite fast, then typing becomes slower and slower, until the text box apparently freezes.

If I paste the long message into a different text field, like the text field I'm writing this message in, the performance is constantly slow. In other words, it doesn't become slower and slower, like in gmail.

So it could look like it's partly a general chrome performance issue with text fields (Firefox does not have this performance issue), and partly a gmail bug.

Did this work before? No 

Chrome version: 51.0.2704.63  Channel: stable
OS Version: 
Flash Version: Shockwave Flash 21.0 r0

Other tabs are responsive, and I can also continue reading other messages in gmail while writing. It's just the text field that freezes.

Same problem using google-chrome-stable-49.0.2623.112-1.x86_64.

I've tried chrome 50.0 on a different computer, and it had the same problem.

I've tried turning off all extensions. Did not help.