The ones that load immediately don't scan the file.
I'll bet Vdub doesn't load immediately.
Have you looked in options for
"full sequence scan when loading mpeg" The default is OFF but...
What are the properties of your file ? what audio ?
Have you tried generating your own MPEG files under controlled
conditions to use ? Encode one yourself .
And last but not least, does it finally work OK after the slow loading. ?