I doubt the field codes are an issue.
It may be an issue of the connection quality of one of the users. The servers have to keep the communication between all users for the updating each users file and displayed image. Have you tried collaborating with the web-app?
File size, not page length should be the other focus. There are a couple of things that can bloat file size. I was surprised at how much a file grew when I embedded fonts. I also played with a second language once, Japanese, and that really bloated the file size.

Applying formatting directly instead of using styles also bloats file size.