With any sort of performance issues you need to identify specific areas which are causing concerns and these areas should usually take the majority of the total time to complete the task e.g. one query on it's own shouldn't be reviewed and optimised however one query which take 25% of the time whereas another 10 take the remaining 75% probably should.
As Frans suggested I would compare specific queries or rather uses of LLBLGen code e.g. GetMulti on a collection or Save on an entity to see how long this takes for each database.
I'm sure there aren't intentionally any bottlenecks in LLBLGen providers for Oracle or SQL Server however it's always possible that the Oracle provider maybe able to be written more efficiently in some scenarios. If you spot one, perhaps as in this case, then you will need to be a lot more specific about where the differences lie so we can help diagnose this further.