It appears that HHH-3482 Closed inadvertently caused StatelessSessions to be 2LC enabled, which they shouldn't be. This causes a decent performance hit. Most uses of 2LC first check Session#getCacheMode, as they should. Others seem to rely solely on the Persister having 2LC enabled.
When 2LC is used, Session#getTimestamp is always called. That call appears to always be surrounded by try/finally. Before HHH-3482 Closed , StatelessSession would throw an exception for that method, essentially disabling 2LC use.
Audit all uses of #getTimestamp and ensure that StatelessSessions are blocked from using 2LC by other means.