When a virtual database (VDB) is being provisioned, Delphix first creates a virtual copy of the most recent datasource snapshot, and then performs a roll-forward recovery to the specified point-in-time using transaction logs. The more transaction logs (i.e. Oracle redo logs) that need to be applied, then the longer the VDB provision operation will take.
One way to minimize the duration of this roll-forward operation is to adjust the snapshot policy of the underlying datasource of the VDB so that the most recent datasource snapshot is close to (if not the same as) the specified point-in-time to which the VDB is created.
Clearly, doing so implies some foreknowledge of the desired point-in-time to which the VDB will be provisioning, indicating that this may be a frequent or automated operation.
If the underlying datasource is a dSource, then the snapshot policy is for the "SnapSync" service which uses the Oracle RMAN utility to capture changes in the source database. If the underlying datasource is another VDB, then the snapshot policy is for "VDB snapshots" which merely create snapshots or restore points in the timeflow of the underlying VDB. Go to Manage > Policies in the main Delphix administrative console to adjust either the SnapSync or VDB Snapshot polices for the underlying datasource...
Please reference the documentation on policy management when making changes to either SnapSync or VDB Snapshot policies.