I noticed the same thing, but disabling the chat feature fixes the problem. Under the chat window on the left hand side of the page is options. Click options and it will give you a chat menu. Click "sign out of chat" and you should notice a performance improvement.
In recent months, more often than not, I have to resort to HTML view and then change to standard view from that, as it is so slow loading. Yahoo is very quick in comparison but has features that I don't like such as opening the next email down if you delete an open one.