Assistance with website Optimisation (Images, Speed etc)

I'm finalising on the last few plugings and functionality of this social media style site but I'm having serious problems with website speeds.

Google insight says I've not optimised images yet SmushIt says I have. Is there any chance someone could explain or show me how I can fix the image optimisation, as well as how to get a little bit extra out of the website.

It's on Bluehost's Business Pro which I know is still shared, but their techs said it should be sufficient to start with and at the moment waiting 4-5 seconds for a page just isn't going to be acceptable.

Weirdly enough on Pingdom I've got a lot of 'waiting', what exactly can I do to fix this.

Much appreciated, love all the support at WPMUDev.