Looking for file-based DB engine alternative to MS Access

My company has been chained to this MS Access '97 .MDB format we've had since the dawn of time. Our schema is hierarchical, with multiple one-to-many relationships. We are at a point where we are looking for alternatives, as the slowness and overall clunkiness of Access are beginning to take their toll on our productivity. Our "modern" methods of accessing the database involve DAO.Net and heavy amounts of hash-based caching. The .NET System.Collections.Generic.Dictionary type has been a god-send here, because without it, I don't know how we would get our work done in a timely manner. We have multiple projects that each have a database file associated with it (sometimes multiple), and we tend to interact them one of two ways: either the DB is created by hand (using our in-house editor), or generated using a program which takes data we receive from another company, in some other format, and converts it to our format. In both cases, our common .NET library loads the entire database into hash tables via Dictionary and resolves the object relationships with code by looking up values in the hash table by ID. When auto-generating the database, we use another set of hash tables to determine whether an object exists already in the cache before adding it. Once we are finished parsing the source data, we start a multi-threaded bulk insertion operation. We do all this because any other method of accessing the database is very slow. I hope I've given enough context to my question: Is there a DB engine out there whose query speed can rival that of hash tables like what I am using? Memory and disk usage are no concern, these DB only exist on developer machines, we convert them to a different format for use with our software. I just want to get rid of my hash tables, but I don't want to sacrifice speed to do it.