Is a serial code quickly than such one transformed to thread?

Is a serial code quickly than such one transformed to thread?

I was made simple and very
complicated experiments too, when take a serial code and put it in one thread
(threads). Execution time increases about two time after that. Id like to know
some explanation. As result, two processors (cores) are not enough to
increase efficiency of series code because we have two times delay (series code
to threads) and some time for synchronization (by events) in some cases.

I have used in my experiments
QuickWin library and Digital Fortran for creation multi-threading under Windows. I'd like to know if such problem will exist if Intel Fortran is used?

Or may be I mistake?Thank you in advance,

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