This very much depends on how you intend adding the video.
If you use a third party to supply the video such as You Tube where the video sits on their server and not your
site. Then the speed will not be affected, as the
site is only viewed on your webpage and isn't actually stored there.
However if you put video on your
site in the form of a file then teh speed will be reduced and the
site will have to retreive the video and prepare it for playing before your
site fully loads.
We hope this is helpful,
Reason8 Support