It seems that we're having problems with the server that the site is hosted on. We're in the process of moving the site to another server, which will resolve the performance issues. This is quite a big job, I'm told, which will take a number of hours. So, until the job is complete (the development team is aiming to get it done by late this evening), it's likely that the site will continue to load slowly.
We're very sorry for the inconvenience and annoyance caused, but please be assured the problem is being worked on. Thank you for bearing with us.