I didn't find any performance problems in the core. There was a problem that causes 9i to quit, but in the forums somewhere. I think it's a view that joins against null in some weird way. That would be in a 4.2 upgrade.
I had about 5 problem queries out of thousands, but until I fixed them the site was almost unusable with more than a few people. I didn't really find any pattern either. One was something like
select stuff from table
union
select stuff from dual
The dual apparently confused it. I did notice there is a dual optimizer in 10g
