This was beyond coincidence that two different customers, in highly concurrent use of an APEX application, identify latch locks and/or latch contention issues with the very same line of the APEX source code.
The PL/SQL block in question was in the activity logging PL/SQL package of APEX. This is the package that writes to the activity log of Application Express, which tracks information like the application, page, user, elapsed time, etc. One of the changes in Application Express 4.0 was a new attribute in the activity log called content length
apex.oracle.com was on APEX 4.0.2 on 22.214.171.124 and this was never an issue
but that's not the source of the latch contention. In a nutshell, Tom suspected that there is a change in the way the parse happens in 11.2, and a bad parse (i.e., failed parse) is more expensive in 11.2 than earlier database versions. Is it a database bug? Maybe - but it also could simply be changed behavior.
For our customers on large, highly concurrent production sites on APEX 4.0, when they decide to upgrade to Database version 126.96.36.199, they may experience degradation in performance for the same application with the identical load. There is now a patch set exception (a "one-off patch") for this issue for APEX 4.0.2.00.07 on My Oracle Support. Look for patch 12404581. The actual patch itself is not database version specific. Hopefully this blog post (and patch) will save some of our customers from future headaches.