When you start burning a DVD, you may be prompted to pick the speed of burning - you should pick the fastest speed possible. Some DVDs have to be burnt in "realtime" however, ie an hour long video would take an hour to burn.
Slow DVD are a common problem for computers, especially computers that are fairly old. Sometimes it is a simple answers that can solve your problem. Try to make sure you computer is not too hot. Hard drive fans overheat, especially when the fan is blocked up against a wall or in a corner.
There could be a few reasons and these are the most common:
-Your computer is running out of memory, so try closing as most programs as possible while burning.
-Your DVD Burner is old. The new ones should burn a whole DVD in about 15 minutes so, if it's taking too long, maybe you should upgrade.