WordPress breaks on big posts and websites?

Does WordPress break when adding lots of big posts?

I am working on designing a website to add wordpress the backend. The website has been around for almost ten years and does email newsletters, so we are adding all the HTML newsletters into WordPress.

There seems to be many problems arising after uploading many of the email newsletters.

For example, some of the category pages now do not load, for some strange reason. Some category pages do load fine, but others do not. I have already deleted everything in WordPress and done a fresh install last week when the problems arose, but now they are happening again!

For example, this page doesn’t load correctly:
https://www.xianzai.com/beta/archives/category/xianzai-guangdong/

Yet this page does load:
https://www.xianzai.com/beta/archives/category/xianzai-dalian/

Both category pages use the same category template, so markup code shouldn’t be giving it a problem.

The main part of the beta website is found at https://www.xianzai.com/beta if you want to click on some of the other categories (located in middle of page next to each “RSS” button)

Please help! What could be causing this behaviour?