Join condition on complex views
Never assume or "belief" the cause for something. Test it. 

In your case run an EXPLAIN PLAN on the statements in question and check if the plan with views is more expensive than the corresponding SELECT using the base tables. 

You can also use SQL*Plus' autotrace feature to check out logical IO done by both queries. 

If that still doesn't give enough information you can turn on tracing and examine the output of the TKPROF formatting utility of the trace file.