Ok, I tried all that and it still seems like it's slow or not working properly. I'll manually check for new mail, and sometimes it'll even say soemthing along the lines of "downloading 10 messages," and then the messages are not there. Are they going into some netherworld lala land or what? Very annoying. This has only been the case since I upgraded to 6. Wish I hadn't done that.
Lol at lala netherworld comment

Well, if it makes you feel any better when I first got my 4S back in Nov. and put my yahoo acct on it, at first it worked ok, but then after about a month or so it just started getting really wonky. Now I rem there was a thread on it along with quite a few other members that were having issues with their yahoo acct as well. Most of us were using the free acct not the paid premium, or what ever it's called, SO maybe because it's not a paid acct then that could be part of the issue on how the email gets handled too? Idk, just a thought.
All this was happening for me on 5.0.1, if that makes ya feel any better lol. But for you, if you just started noticing a difference after updating to 6, then something is different. Btw, I chose to stay on 5.0.1 myself. Never even moved onto 5.1.1 lol.
Did you try to use the fetch method instead and that didn't work well either?
(oh also just to add, prior to getting my 4S I had a bb bold9650, and even with that my yahoo email was a royal PITA!! Lol so I knew it wasn't just the (mal)function of my iPhone)