probably depends on the speed of your computer, but on anything reasonable modern it will only take a fraction of a second to do the conversion.
if you do it on the fly you dont need to make any assumptions about coverart names/types you can simply use whatever is specified for the film you click, you also dont have to remember to add a file to the folder whenever you add a new film.
also why waste sapce for all of them bitmaps when its really being used as a tempory file?
doing it via a plugin is very easy (it took me all of 5 mins to write...), no extra software is needed (it converts and scales internally) and works without any issues.
using my plugin i cant tell any differance when i open up a film, appart from the fact that the dvd cover shows up in TT