You are not alone. Even if you optimize all your images, you will still score poorly on Googles PageSpeed tests. Squarespace has a bunch of backed code that we can't optimize. They say they are working on it, but whenever this topic comes up all we hear is crickets, or "we are working on it." No timeline from Squarespace as to when this will be fixed, which is too bad.
Bottomline is, if you want a blazing fast website TODAY, then Squarespace is not the superior choice. Maybe in a few months they will release an update that solves this pain point.
If you are on Squarespace 7.0 then your site might be doing alright in real world tests, which you can see in your Search Console under "Page Experience" Google said themselves that they will take real world data over testing tool data. In my experience 7.0 sites are faster then 7.1 in the real world.
If you care to watch my rant and see some examples of Squarespace site speeds then you can check out my video about this B.S. : https://youtu.be/BGNmTb1G5a8