If you need to convert a very big Firebird 2.5 database to 3.0, or many databases during the short period of time (for example, overnight or during the weekend), you will definitely find useful this article: it describes how to speed up the backup-restore process by 25-30% with the streamed backup-restore process.
Please read Fast conversion of Firebird 2.5 databases to Firebird 3.0.