Since there is nothing better than iDVD for creative menus and scene selection I am to the point I just need iDVD to work no matter how slow I adjust the speed.
For my (2012 MP) running 10.12.6 I might just replace the internal optical drive, use the Disk Image option and keep speed at 2x’s to have any chance of getting successful burns.
Old Toad has mentioned how they types of files you put into an iDVD project can also effect the chances of a successful burn. In other threads like this he has graciously answered others questions related to the files/ratio aspects iDVD prefers. Lots of great trouble shooting tips out there and great to get some guidance as to the best kind of internal/external optical burners out there to eliminate hardware from the variables that cause unsuccessful burns.
My other (2006 MP) cannot and has not had any OS upgraded since Lion. This old Mac is what I have relied on to burn iDVD’s sucefully by using Disk Image and 2x’s.
A 4X drive will run at the same speed with FireWire or USB if the computer's USB is not being heavily used by other devices. If USB hard drives are in use while burning a DVD via USB, it might slow the DVD burning.
If you have a choice, look for a DVD drive with USB 3. If you ever get a new Mac, it won't have FireWire, but will support USB 3.
An analogy to go along with what Malcom replied might be…..Your in a car that cant go faster than 40mph--- You have to choose a highway to escape a hurricane. You can drive 40 on a congested USB 2.0 road where other USB devices can can weigh you down or drive 40 on a more open FW-800 highway. I just want to give my self the best chance of getting to my destination no matter how slow. I want to escape hurricane multiplexor!!