Problem with Sony Vegas reverse video

Hi,

i recorded some footage but its missing part of the video and only the audio is complete. The video is a guy talking so i thought it was a good idea to replace the missing part by duplicating the part i do have and adding it instead of the missing part, but since the ends wont match up i want to reverse the added part so both ends will match and look natural. However, the reversed video plays with a lot of lag and not as even near as smoothly. Is this normal or there is a problem with either vegas or my video?

Thank you.