Studio Mesa, a website that sells premium Squarespace templates, explains:

Even with all the optimization in the world, Squarespace websites are doomed to a poorly-performing status due to it’s built-in CMS. To put it simply, Squarespace uses a Content Management System (CMS) to make it easy for users to build sites. Instead of writing code, you’re able to visually drag-and-drop blocks. This is great for designing, but this ease is what causes the performance to drop catastrophically. This is bad news for virtually ALL Squarespace users, no matter the version, amount of content, or efforts to optimize.

Even so, today his site is on Squarespace