As to address why your renderings all of a sudden starts taking a lot longer,
is a possible windows update could have thrown things off for Vegas and can
be corrected by uninstalling any new updates. Or a display driver/GPU Card
update, in which a driver Roll-back would help.
You can try a fresh Vegas Re-installation also and see first!
Steve Rhoden (Cow Leader)
Film Maker & VFX Artist.
Owner of Filmex Creative Media.
Samples of my Work and Company can be seen here:
https://www.facebook.com/FilmexCreativeMedia