15 minutes for a DVD is possibly acceptable but I would expect a bit better from the computer you describe. It depends a little bit on the configuration. How long are your videos? Are you extracting photos? Is there any promo video?
We can produce a seven minute DVD with 1080 photos extracted from the video in about 12 minutes or a USB in about 10 minutes but our computer is slower than yours. I recently configured a brand new Notebook and it was taking 30 minutes for the same USB our Notebook pumps out in 10 minutes. Turns out the brand new AMD processor was three times slower than our two year old Haswell 4th Generation i7 CPU.
HD at 60 fps is overkill especially when delivering a DVD which is SD at 30 fps. This will increase rendering times as it has to convert 60 fps back to 30 fps and it doesn't simply drop every second frame, it interpolates the footage so it takes a long time. I'm also assuming you're in the US and you're delivering NTSC DVD.
A common issue affecting rendering times is having your Vegas template set different to your camera. I recommend 1080 at 25 fps for PAL DVD so your camera and your Vegas templates should be set to this. For NTSC, its 1080 at 30 fps.
TandemVids already allows 1080 (and 4K) on Blu-ray or USB but DVD will always be SD.
Mick