Hi there,
I used the file and imported it into my local installation and it is working ok. But the problem is that the page is too long and there are a huge amount of the sections available. That is why it needs lots of resources to do changes and indeed it will be slow. In my case the preview shows but in loading mode and clicking on the elements brings the options to change:

I am afraid this will be inevitable even in the new version which is on the way as your page is too heavy to handle. And your server resources are limited and it seems that you have a low timeout time.
The only thing that I can think of is to make your server have much more PHP Memory Limit and increase the PHP Timeout to at least 300 seconds. You need to contact your hosting service provider. But even that might not work if you are on a shared hosting and the performance is low.
If I were you I would have a separate installation on my local machine where I can change the memory limit and stuff with no problem, then I would do my changes locally and save them as the csl
template and import them on the server.
Unfortunately, there is no other way for you to this specific page.
Thank you for your understanding.