AS did this for a couple of years on their flights: pre-boards, First, High Elite, Regular Elite, and then ALL ABOARD. Inevitably, some passenger in row 6 would block the aisle for 3 minutes while stowing luggage, getting a pillow, blanket, etc., etc. So, what AS did is split Y into 2 groups: row 15+ and then all rows. This helped considerably.

Basically, you need to get a group of people to the BACK of the plane first. Then, you must pause the boarding for, say, 3 minutes. Finally, board the rest of the plane. It works pretty well. Optimally, you probably want to board like this: rows 20-25, rows 6-10, pause for 5 minutes, rows 11-19.