most simple solution is to switch the filenames of the filestream node. if you like to have blendings between two video sources you will nee two filestream nodes… there are several tricks to pseudo blend with just one filestream, but they are more complicated to achieve… how should your video switch look like?
The old school trick of avoiding latency when switching files (rebuilding render graphs) would be to concatenate all your clips into just one file and then doSeek to a keyframe.
If you don’t have more than a handful of clips you can also just open all of them and play only one. (this will be limited by your computers resources, it might be interesting to find out where these limits are)
and yes, time to open a clip does depend on the codec you use.
Picvideo is quite a favorite here, XVid seemed quick to me, too - as long as you don’t scrub the video.
Then (if you’re desperate) there’s also the high-performance possibility of playing preloaded textures directly from RAM…