WP Site Speed Very Slow and Sporadic

We have a lot of WP sites, and we are starting up a new website: https://powerbanksouthafrica.co.za/, but the site is having issues with speed.

It takes a VERY long time navigate anywhere. It hangs when you click on any button and then loads and loads with nothing happening for a long while.

The issue is very sporadic; sometimes it seems to work fine and it seems fast enough, but the majority of the time it is very very slow.

I have tried all the basic speed tests and recommendations;
I tried a different theme, I uninstalled and tested the plug ins separately, I use a cache plug in, but none of it seems to work.

I use all of these same plug ins and the same theme on many of my other sites and none of them have this issue.

I had initially thought that is was a problem with the server, but I contacted the hosts and all is fine from their side. Even speed tests such as https://tools.pingdom.com/ and https://gtmetrix.com/ don’t indicate any issue in the speed.

But it still takes forever to load! It often just loads for so long that I just manually stop and refresh out of frustration. It feels like I’m using dial-up internet, but only on this particular website.

Please can you advise if there is anything else I can try.