It is common for individual websites to go very slow or not load at all if the site is experiencing heavy traffic (lots of people visiting the website at the same time) or that the server or website is being changed at the time you are visiting.
Therefore it is worth trying again as you may well find that the Yahoo mail site is now back to full speed. However, if you find that you are still having the same problem and it is still only with the Yahoo mail service, then it could well be worth trying to contact Yahoo mail.
If you are very unhappy with the fact it keeps being so slow it could be an idea to switch your email account. You can open free email accounts with Google; in the form of Gmail. It is very simple to set up and as Google.com is the biggest website in the world and it has the best servers. As a result they should always stay lightening quick while you check into your email account.