The REST services architecture is stateless see (REST WS) so it makes no sense to have the a rest service with
InstanceContextMode = PerSession.
The performance improvement using PerSession
(however concurrency issues may appear) is for
SOAP web-services.
Answering your question I believe that is a fortunate series of events (db connection pooling, database cache, etc) that you see a performance improvement.
As InstanceContextMode.PerCall
is the stateless mode for WCF it is also instantiation mode of your rest service, even if you specified the PerSession
as InstanceContextMode
.