How to make your site run quicker?

Hi, New to the WordPress forums here so bare with me! Our main company website (birminghamtelecom.co.uk) is very unresponsive in terms of page load times. Whether its the customer viewing the site or us editing using the back panel, it just seems to take ages to load. We use a reseller hosting account with Hostgator, and for the majority of the time the rest of our sites don’t seem to be running too slow (buyaphonesystem.co.uk) for example.

I’ve done a test on GTMetrix and it mentions about specifying image dimensions which is fine, but is also pulling up information about defer parsing of javascript. Is there anyone that could take a look at the site to see if theres anything that can be done to speed things up?

Kind regards

Jay