delay problem in playing a part of video with mplayer

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C#5.0

I learned how to use mplayer in C# by this article : Simple Mplayer Front End in C#
when I use -ss 25 -endpos 10 mplayer should displays ten second from second 25 until the second 35
but it just displays 5 last seconds( it displays from 30 until 35)
i think because of the video is large (about 1.5 hour)it has delay.
how can I solve delay problem?
thank you in advance.