I have Roxio Toast 11 on Mac OS X 10.9 (Mavericks), and was trying to burn some Blu-Ray BD-R DL disks on my new Pioneer BDR-XU03 burner tethered to the USB 3 port on my MacBook Pro Retina. Unfortunately, even with fast Verbatim CD-R DL 6x disks, it took over 2 hours to burn each disk, and another hour to verify the 50GB of data. This translates to a write speed of between 1x and 2x.

Eventually I found the culprit - Mavericks' App Nap and PowerNap features. Turning off PowerNap in System Preferences, and right clicking Toast Titanium in Finder, and in Get Info disabling App Nap fixed the issue. It now burns at 4x (about 45 minutes for a 50GB disk) and does not stop mid way. It used to pause reading / writing the drive every 1 minute if Toast Titanium was not the active app.

Clearly this is a major issue that should be stated clearly on Roxio's web site, but as far as I know it is not.