I haven't done much to concern myself with speed and performance other than follow squarespace guidelines
I've made some images as low quality as i can while ensuring they don't look poor -- that's about it.
One thing -- I've taken some images and overlayed them with opaque white to make them less crisp so that the black font would be more easily seen
You can do some free photo editing here -- Pixlr
Another beginner tip -- I was able to add some CSS code to highlight some area of text -- all i did was google what i wanted to do and some sites have the code right there -- you copy and paste the code and voila! -- you did it. Key thing -- you need to know what you want to apply the CSS code to. You want to add "Squarespace Block Identifier" to your browser -- it ID's the blocks on your webpage -- its a great productivity tool.