Unable to access site - slow load time

I am working on a clients site https://www.inspectwollongong.com.au the issue is I cannot load the site on my PC and on mobile is very long time to load. HOWEVER if I use Gtmetrix or Google Page Speed Insights both are showing fast load times. I do not understand why the above show fast load times yet will not load on my pc and loads slowly on computer?

I need the following:

1. Fix issue
2. Explain why Gtmetrix and Google show fast load but not working like this for me?