If you get 1500 emails a week, you're going to spend some serious time processing them. Even at 30 seconds each, you'll still need more than 12 hours.
Without knowing where the email is coming from, it's hard to suggest ways to deal with it, but that would be my first priority in your position.
Do you have good spam filters to get rid of the true junk before you ever see it?
Do you use message rules or other tools to route mailing lists, automatic notifications, log reports, and so forth to folders where you can process them in bulk?
In most offices, lots of mail is really just some form of status report, copied to everyone who conceivably might be interested in how the sender is spending their time. "Bob, can you get me the Smith Corp. sales figures?" is an action item for Bob, possibly a vaguely useful piece of information for Bob's boss and assistant, and a complete waste of time for everyone else. Can you use collaboration tools and/or change the office culture to cut down on this kind of email?
With meeting notes, can you "pre-process" them during or right after the meeting? Flagging action items and so forth is a lot easier when the subject matter is fresh in your mind. Or, conversely, are you attending lots of meetings where you *don't* end up with any action items, and if so can you figure out a way to avoid such meetings altogether?
For the amount of material you have, your processing time doesn't sound out of line. If you can't cut down the amount of material, perhaps you can change how you view it? That is, even if you weren't using GTD, you would still have all those emails coming in, you would still need to read them and decide what to do with them, and doing so would probably take more or less the same amount of time, possibly even longer. Only you wouldn't call it "processing my inbox," you'd call it "reading my email." My point being that processing time is *not* unproductive "wasted" time, it's a (hopefully) more effective way to manage tasks that, GTD or not, still have to be done.