I am having the exact same issue.
My site is only 3 months old. When we were building it, the designer was extremely fast and responsive.
As we’ve added more and more pages and CMS collections, it’s become slower over time. And now it’s so bad that it takes 30-90 seconds to load another page, up to 90 seconds to publish a page, and lags and freezes anytime we are editing a page.
We also have top of the line computer specs (i7 2.2 GHz, 32GB of RAM, SSD Hard Drive, NVIDIA GeForce 1050 Ti graphics card). Also tried using every browser imaginable.
Hoping for a resolution because it’s becoming unusable to work with it.