If you go for places like the heart, Temple and kidney they will cause pretty much instant death. If you go for the suprasternal notch(you can thrust downwards into it and cause damage to the vessels near the heart) you will need to wriggle the knife from side to side to cause more damage but still leave it in.
Even if you have gloves you don't want to get blood on them, blood is evidence. If you use the right technique it will be fast and you will be walking casually away with no murder weapon to get rid of or blood on you. You should be wearing gloves anyway though.