Just because it worked before doesn't mean they have not changed something on the site that is slowing things down, maybe even a third party advertising banner or a change in the HTML coding. SInce this is unique to an individual website if you could give us the URL we could see if the problem is specific to the site or unique to your configuration.
If it is unique to your configuration, there are any number of possible reasons this could be happening, but one of the most obvious suspects would be some extension you are running in Safari — most likely an ad blocker. There are lots of options for testing that out but the easiest is only available in more recent versions of Safari. But in any version of Safari in the last few years you can go to Safari > Preferences > Extensions and individually turn off any and all extensions you have installed.
Additionally be sure you don't have any ad blocking apps running outside of Safari.
