No worries, this site is by far the best tool i’ve used for grammar and I want it to be as good as possible.
I’ve had a deeper look today and found two other things that would speed things up a lot:
<link href="https://fonts.googleapis.com/css?family=Noto+Sans" rel="stylesheet"> <link href="https://fonts.googleapis.com/earlyaccess/notosansjapanese.css" rel="stylesheet">
The site is using both normal Google fonts and a old unoptimised preview version. It looks like the main font is loaded using the normal version and the Japanese font is loaded using the old version. I had a quick look on Google fonts and found the Japanese font you are looking for: https://fonts.google.com/specimen/Noto+Sans+JP . If you switch to this it should cut around 7mb off the page load.
A lot of the images look to be uncompressed as well, compressing these could speed up page loads (saving you money in server costs)