I can see that each account keeps it's balance of lumens. Yet there should be a place where the rest of assets that belong to an account is preserved - the API for Account Details returns a list of these.
My question is how heavy is that operation? Is the data come from the database (where it's indexed properly) or there is a need to look into the XDR files, by a chance?
Each account trustline is represented as a record in a trustlines table located in the Core database. The trustline itself has a balance field that contains current asset balance for that account.
"Find asset balance for the account" is rather cheap operation considering that the query involves a simple join over trustlines and accounts tables on indexed fields.
If you need to query this information very often, consider running your own Core node (in no-validation mode in case of limited resources). In that way you will be able to get data directly from DB, omitting the penalty of serialization/deserialization and network lag in the case of Horizon API query.
