I run firefox on windows. I can edit with squarespace fine up to a point and then it starts lagging. Worst of all, it affects the rest of my firefox tabs. It's like it hogs all the resources. As I typed this, I had to wait for it to catch up every few words so it must be something with their back end. I tried it on edge and it was exactly the same. I have to wait for things to catch up just to click a button.
My pc isn't a slouch. I run a ryzen 5800x 8 core with 64gb of ram and I have a 24gb rtx 3090