If Yahoo is loading slowly, it's probably because of one of two things:
1. There are a lot of people on Yahoo at the moment.
2. Your Internet connection is slowing you down.
It's probably the second reason, but not impossible that it's Yahoo's own 'fault' for running slowly. A website as big as theirs shouldn't really be affected by the number of people that are on the site at any one time.
But there can be times when their website will be so busy that it's driven them over their normal capacity, which will affect some people's ability to use the website at normal speeds. This could happen during peak times - when, for instance, the majority of America is awake - or if they run a very popular and exclusive news story, which will drive traffic to their site.
It's also possible, though, that your Internet connection is the problem. You can run free diagnostic checks online to see what kind of upload and download speeds you're getting, which should give you an idea of if the problem lies with Yahoo or your own connection.
If the speeds are lower than you'd expect, then get in contact with your Internet provider and see if there's something going on on their end that they can help you with.
Or you may even want to consider switching supplier altogether!