First of all, I want to mention that we totally understand that it takes so much time to create the page and now that does have the problem it is frustrating to start from scratch.
Unfortunately, we do not have any tool or mean to find out why this is happening. The fact that it is happening only on one page just shows it is not related to the plugin itself as if it was the other pages should have the same problem.
But as it is a very long page the server factor comes in that might have the issue. So, in general, there are 2 possible cases:
The server cannot handle that big of a page when in the Visual Composer. Which we already asked you to move your website to your local machine or another server to test.
There is an element which is causing the issue, which it is not possible for us to check and you need to double check the case.
One thing that you can do is to install the duplicate post plugin and copy the page, then in the newly copied page try to remove the elements one by one to see if there is an element which is causing the problem. In this test, if you see that after removing certain amount of the element no matter which element that is, the page works faster it is 100% server problem not being able to handle the elements from a certain point.
Please consider that even if there is a bug of some kind in the builder (which I doubt as there are many many people using this plugin) we will not be able to fix it, we will be able to only report this to the developers of the Visual Composer to be considered for the upcoming releases.
Anyway, if you are trying to start things from scratch I suggest that you check the Cornerstone as it is the go-to builder for X.
Thank you for your understanding.