First, a big
Welcome Aboard,
Chriz!
Now, about those messages...
1. Do you have the two programs ( Mail, I assume ) set to delete messages after downloading them?
2. Are both computers
always running at the same time?
3. Are both iMacs
always running Mail?
My suspicions are that which ever Mac/Mail downloads the message, it will not be seen on the other. Or one is set to delete the messages and the other is not.
As for the speed difference, I can't help. You may have different apps/filters/etc. running on the different machines.
Are you using a router? Or does one machine access the other to get to the 'net?