I'm sure someone from Squarespace will tell me that as a platform, Google treats them differently than the rest of the world - or that in fact, SS is 'special'.
Google not to long ago released Core Web Vitals. They have a good explanation of what this is here: https://web.dev/vitals/
Don't be fooled, web.dev is Google, not some third party.
Guess what, Google doesn't treat Squarespace any different from any other website - whether it uses custom Javascript, PhP, Asp.net or other code vs. a builder CMS like SS, Wix, Weebly. They are all the same in the eyes of Google.
So