The user in this thread experienced a similar issue and increasing the memory limit fixed the issue. In case you don't know how to increase WordPress memory limit, please see https://theme.co/apex/forum/t/troubleshooting-increasing-php-memory-limit/16935
Upgrading the PHP helped another user having the same problem: https://theme.co/apex/forum/t/cornerstone-superslow-to-save/50678/4
If you experience the issue only in that page, please save the page as a template and then load it to a test page. Then in the test page, remove 1 section then save the page and take note of the time it takes to save. Repeat this step for the rest of the sections. This will tell us if you can add lots of elements in the page. This might be connected to limited server resources like CPU power and memory.