As any other db4o operation delete is dependent on the performance of a storage location. This can be easily compared with a database stored on a hard drive and in RAM:
Delete test: RAM disk
Store 90000 objects: 5973ms
Deleting 1 object of depth 3 on a RAM drive:
Deleted all objects: 5249ms
Delete test: hard drive
Store 90000 objects: 5043ms
Deleting 1 object of depth 3 on a hard drive:
Deleted all objects: 7475ms
The general rule is: the faster the drive is the better is the performance.
Download example code: