I trying to build out a service that holds user's monero private keys. But the issue I'm having is how does one update their balance of these private keys? And no I cannot use subaddresses for my use case as each user has their own unique private key. The solution I came up with was scanning each monero private spend key manually one at a time and updating the balance like that. But that's not going to really scale that well if I had 100k private keys to scan.
What can I do to solve updating monero balances of 100 user's private keys?