Speedy Response
I assume you are using a lot of Scripts running in the background for the sake of beautification but the Advertisement Scripts running in the background are a big menace hindering your page response speeds.
As this section is a platform to give you(OUR BELOVED OLDEST MAIL PROVIDER) some suggestions, my suggestion to you is to prioritize the basic operations on the page which are the purpose for your provided services and then load the page Scripts for advertisements.
Scrolling the Mail Section is too 'Glitchy' to wait for the response. So I suggest you to make the interface and scrolling palette a simple responsive one, which JUST serves THE Purpose.
I Expect these in your future updates.
By the way I've logged back on Yahoo Mail after quite long time which I left for these above reasons.
But now I'm here to stay & for future regular use.
So I'm sincerely expecting these changes.