A couple times I started to burn a DVD , and about 15 mins went by before even 1% appeared in progress bar. Drive would spin/stop/spin/stop. "LBN" indicator (lotsa binary nonsense ??) was poking along at intervals of less than 100. I knew there was something amiss.
From then on , I make sure the working files are on my 2nd HDrive, since the burn program is on 1st, as is the destination (or temp folder for burning). After initial baby steps (not always) it takes off in speedy fashion.
Not sure what those slow burns were about (aborted them), but dont have the problem anymore.