MySQLJDBCDataModel vs FileDataModel

[Mark]

I've read the source for FileDataModel and it suggested using a JDBC 
backed implementation for larger datasets so I decided to upgrade our 
recommendation system to use MySQLJDBCDataModel with 
MySQLJDBCInMemoryItemSimilarity.

I've found that the JDBC backed versions performance is actually worse 
that FileDataModel and FileItemSimilarity versions. Should this be the 
case? Which versions are most people using out there? Any recommendations?

Thanks