I don’t have a solution for you, but I wanted to let you know you aren’t alone. I’ve noticed a drastic slowdown when using the editor too (the normal page view speed is normal and unaffected), in scrolling the sections, but also end when selecting options or even adding text to a block. This started about 2 weeks ago.
I’ve done a lot of testing and spent hours with Shopify support. I have duplicated the editor’s lag on different computers, on different networks (e.g., home wifi and cellular), and different stores.
Shopify support wanted to blame apps too, but I had the same experience on a new app-less store, using Dawn, only Dawn sections, and no images, video, etc. In other words, a completely vanilla experience.
Shopify support eventually acknowledged they could see the lag in this empty test store once more than 12 sections were added. The sluggishness got worse as more sections were added, and nearly unusable at 25 sections.
The claim was that everyone will see lag as more sections are added, because of the editor’s overhead growing, but that each person will experience it differently based on their computer and internet. I get this, but what I’m experiencing now is much different than what I experienced weeks earlier.
It got kicked up to the developers.