Why it increase the processing time on un-parallelization part with OpenMP?

When you specified OpenMP or Auto-Parallelization option on compile, the compiler links the library for parallelization whether parallel directive or not.
The functions of library for Parallelization differ from the normal functions in that have the lock routine for that other thread limit access to resources.

If it calls the functions of Library for parallelization on the un-parallelization part, it runs on one thread, of course. Therefore, it will not wait for other processes for lock routine. But, it needs a little bit processing time. Because the functions of library for parallelization have to make a decision for whether should do or do not the lock routine. Please note that.