Yahoo Mail is slow, Period ! - You don't want to change application (Mail client or browser?) - But I found Firefox solved that problem, as well as faster response times on other sites that loaded slowly. Worth a try, and you don't need to uninstall IE.
Slow is always relative. Like the frog being boiled, I've been using Yahoo! Mail for so long (since the 90's) I don't have anything to compare it to.
I'm pretty sure that is is just Yahoo! Mail that has the issue and it is only on my laptop. While the two are probably not connected my laptop is in the process of dying.
YM seems to be responding fine on the other systems in our home. This is going to be one of those, "Did I change a setting or add software on my laptop that is causing this?", hard to figure out problems.
I'm thinking maybe it is how YM is programmed. I know there is Java, HTML5 (I believe Apple's web site uses a lot of this and loads really slow in IE9 on all of our systems), and other web programming languages. Maybe YM uses one that is slow just on my laptop.