In Oracle Database 10.2.0.3.0 repository catalog, RMAN seems to run very slow when the repository catalog has thousands of entries in table RSR.
Oracle Metalink shown a bug, and resolution is to create an index
Bug 6476935 - RMAN Resyncs may take a long time to complete
Version affected : 10.2.0.3
Version fixed : 11.2
Affects : Catalog Schema
Workaround : create index rsr_i_stamp on rsr(rsr_sstamp, rsr_srecid);
Someone indicated that using "alter session set optimizer_mode=rule" will help, which I have not tried before