The main reason why Squarespace sites can load slowly is if you've included external scripts and widgets in your sidebar that must load before your page can be rendered. When you embed content from 3rd parties, each 3rd party server must be contacted to download the presentation for your page before it can load. Removing 3rd party widgets will speed up your page load times greatly.
If you have included large images (80K+), this can also adversely affect page loading.
Note: We have a page here that lists free and paid options for image editing/creating.