The data files are large. Initially i did consider storing them in memory, but discarded the option due to large size of the files. Account file is 1827775ely tiv79 byte, which varies daily, but remains more or less of the same size. Currently holds 62394 records.

This is indeed relatively large, but most probably small enough to fit into memory on a decently modern computer. That's what I would try anyway. Especially if you can decide to store in memory only the part of these files which is useful for your process.