My SEO tool says all of my page loads are WAY too slow

I invested in a 3rd party SEO service. The service is telling me that my site and its pages are taking WAY too long to load despite not having a heavy 3rd party code component.

It says I need to reduce unused CSS 
Reduce unused javascript
Eliminate render blocking resources
Serve images in next gen formats

How can these items be fixed?

I'm going to have to switch platforms because this is causing us to pay higher google ad prices.  I manage 3 corporate squarespace sites and the cost is becoming outrageous