I have a database of about 100 million addresses (all starting with 1) and private keys to them. How to check them for possible balance? How can you find lost addresses? Or maybe someone knows how to do it right?
100 million? What are you trying to do? That seems excessive, there's likely a better way to do what you are trying to do unless you're hoping to brute force keys, and if that's the case good luck. Your best option is likely to create a database of the current UTXO set indexing all the addresses that the UTXOs belong to, and then write a simple script to compare those addresses to the addresses in your database.