Just to share - as a warning:

Config: WinNT with Application & 8i server on same box. We store Word docs, so have to run anti-virus, McAfee in this case.

Setting event 10046 for the session from inside the application caused it to run 100x slower; server becomes CPU bound by McAfee. Trace file full of SQL*Net waits.

I don't like my reasoning, but Oracle Support couldn't do any better: Somehow the combination of the application layer (?!) and McAfee slowed down the writing of the trace file, somehow this was reported as SQL*Net waits, which were written to the trace file. The slow writing of SQL*Net waits to trace was reported as SQL*Net waits, which in turn were written to the trace file . . . ad infinitum (almost).

Setting event 10046 outside the application only had a 30% impact on performance. After turning off McAfee, setting event 10046 inside the application had only a 2x impact - negligible number of SQL*Net waits.