It happens over time. Give it a year (or even 3) and you'll notice a huge difference. You might think "why keep the same OS for 3 years" but you get the system pretty tailored at that point. Linux doesn't really need to be re-installed, the benefits are minimal and such. dpkg however sucks donkey balls after a while, you wait a minute or longer (3-5) for that damn "reading package database" crap. It can get really annoying and frustrating when all your doing in one package. So you can speed it up, which really really helps.
I wonder if somewhere there's a comparison of how package managers perform.