We know that the bitcoin distributed ledgers is stored in a serialized blockchain based on the transactions' chronicle order.
The current size of this database is about 100 GB. How can they search easily within this database? For example, if I got a paper-wallet previously loaded with some bits from Xapo, and I try to sweep its bitcoin value into a copay wallet (different from Xapo), it takes about 75 second to search & get its balance!
So, how can they search in this large serialized database within this short-time? I am asking about the technology behind that!