When you compile with OpenMP enabled .AND. specify one thread, the code generated will still use the threadprivate access methods for dereferencing threadprivate variables (a few extra instructions per dereference).

Therefore any slowdown due to threadprivate will be visible in the one thread OpenMP produced application using threadprivates. When you observe a slowdown when going from one thread (in mt generated program) to multiple threads then suspect adverse memory interactions (cache evictions). Or, perhaps, making a parallel version of a short loop that cannot ammortize the thread start/stop overhead in a cost effective manner.

Can you supply a small code example?