The performance version would be a list of public keys sorted by their account id with their balance id, which would take 34.34GB. This would have search performance lg(number of accounts), very fast.
I still think you need to sort by account id not public key
But that's what I wrote.
and of course there are Nxt accounts that actually don't have a public key associated with them
These would have to be treated separately, another list for uninitalized accounts, not a problem.