Hi Zach thanks for posting in the Lucid Community! Lucidchart is optimized for Google Chrome so we generally recommend that browser for optimal performance. Having said that if you are seeing drastically different performance in Chrome and Firefox it may indicate another browser specific issue or bug. 

A large document may use more browser memory. If your documents are very large and you have many other Firefox tabs open or if you have add-ons or configuration settings in Firefox that may be using up/limiting available memory this might explain why you are not seeing the issue in Chrome. 

If the issue persists I recommend starting Firefox in Troubleshooting Mode (formerly known as Safe Mode) and opening your document. This often improves performance by disabling your extensions and other customisations. You can find instructions for Troubleshooting Mode in this Firefox Support article.  If that helps it's likely that an add-on theme or other customisation is causing the lag. Please also make sure that Firefox is fully updated.

 If that doesn't help would you mind submitting a Support ticket using this form so we can help you further? Please include the URL of your document so we can take a closer look. Hope this helps!