Hi Ian thanks for posting! There is not a hard limit on how big a Lucidchart document can be although having many page tabs large canvases on each tab and a high number of shapes on each canvas all increase the time it takes time to load and update when changes are made.

However the fact that you are not seeing this issue in Chrome suggests that the issue is more browser-specific. A large document may use more browser memory. If you have many other Chrome tabs open or if you have plugins and extensions in Chrome that may be using up memory this might explain why you are not seeing the issue in Firefox. 

If the issue persists I recommend opening your document in a new Chrome Incognito browsing session. This often improves performance by clearing your cache and cookies and disabling your extensions. Hope this helps!