Slow datapump exports on 12.1 for schemas in one database but another almost database

I have two very similar databases running from the same 12.1.0.1.0 oracle home on the same machine, on the same disks with the same memory allocation etc. These  databases/schemas are for different customers using the same application, so although the schemas definitions and data contained is not identical, it is very similar. (As described below, even the datapump startup is massively slower, so I don't think it's a matter of a difference in data/data definition)

Each database has about 5 schemas of approximately the same size. On database A, the datapump exports take on average of about 2 minutes. On database B, the exports take on average, about 20 minutes - even for extremely small schemas.