Don't know about the slowdown, but your trick wouldn't really work anyway since you'd start out without the vault suit 'n' Pipboy (and probably in the wrong place on the world map too, i.e. outside Arroyo, not at New Reno).
This happened to me several times but don't know really what was the problem sorry.
Like Per said you must enter arvillage map to get vaultsuit and pipboy(movie must be played) and you can't use many weapons in tribal outfit .
But there will be no worldmap problem since new reno map is connected in city.txt, engine will know where to put player(only camera will be wrong - on start position)
You can play vsuit.mve in the map script script to get the vault suit and pipboy. You can also make a loader with FO2LC, select the starting appearance (hmjmps/hfjmps), and also enable pipboy on start (FO2LC will patch the engine in RAM to do these).
Edit: If you play this MVE in the map script (it's a single black frame made into mve as a placeholder), you get vsuit/pipboy and the movie doesn't show.