If they're going to insist that 900,000 people are eligible for the next round, they NEED to have a pre-registration mechanism. You have 80,000 slots available at Mass Vax sites next week? Great, 120,000 people who preregister are randomly picked. Then, 780,000 at least don't have to spend hours hitting refresh or filling out forms, waiting for tens of thousands of minutes in waiting rooms, or looking at Skippy the Questioning Squid. Then 80,000 take the spots, and 40,000 are disappointed. Any extra spots are opened up the next day, but people are told that they'll be VERY LIMITED in terms of location and times - if you're picky, don't bother, and just wait for 5 minutes.
The Natick Mall site isn't bad IF you can get a time. It asks for prior vaccination info and birthday, and then you get a "all full" or "reserve a time" message - no insurance info, etc. If I recall, the next page is the speed-round, where you select a time as fast as you can. If available, click to get to the latest day to avoid people who just click the first day. Don't take the first time you see - click in the middle where people might not take a time slot. After that, I think you have ten minutes to make sure you qualify (ie 65+, 2 conditions, medical staff, etc.), and fill out your address, phone, and insurance information if applicable. My memory was a little bit hazy as I was trying to book two appointments on the same day on different computers, so I was whizzing through everything.
It's crazy that a week or so ago, PrepMod was asking for all the background info for Gillette and Fenway in advance. They should have learned from Ticketmaster and know that you reserve a time, and give people 10 minutes to fill out all forms AFTER that.