Why is Mail 6 slower in ML than Mail 4 in SL + bugs?

Hello, I have a new iMac 27 with ML 10.8.4. No major problem whatsoever. Actually, Mail 6 is truly getting on my nerves. I come from SL 10.6.8 in a 2006 white Core2Duo iMac. My Internet connexion is 20 mbps cable. On SL, Mail 4would download emails at speeds between 1.5k and 2.8-3k. Mail 6 on ML downloads at speeds between 200k and 1.5k. Customer service at Apple don't know what the problem can be and cannot think of something to do. I cannot accept these slow speeds. Anyone has a cule? Downloading a 10mo image can take almost 2-3 minutes. While I'm at it. Other bugs in Mail? New mail alert makes itself heard only when Mail is running in the background. When in Mail, only the notification tells there's new mail. Plus the no new mail alert is so faint, it has no use. If it were not for Mail 6, my experience with ML would be 100% satisfactory. Can't go back to 8mbps high speed Internet.
Thanks for helping