Why on earth do you want to save a few minutes and then end up with discs that are unreadable? Why are you focusing on speed instead of quality? If you want speed, use one of the recommended brands of DVD+R media. I run with Verbatim and Memorex +R (RICOHJPN) and consistently run at 8x.
If Nero is reporting 2.4x with DVD-R media, it probably is a bug, since DVD-R will only run at 1, 2, or 4x. Check the latest 6.0.028 version if you haven’t updated
With PoweRec on, the drive checks a media catalog in the firmware and matches the vendor ADIP with whatever the factory has determined to be optimum recording speeds. If the media is rated at 4x for DVD-R by the vendor, but the media tested by the factory is garbage, the factory setting may only allow for 2x or 1x. Always run with the latest firmware, since these media catalogs are updated with the firmware. If you’re running at 1.02, you may get your problem solved by flashing to 1.03 or later.
With PoweRec off, it looks at the vendor speeds in the ADIP and sets the maximum speed accordingly. If you still only get 2x, that may be on the disc, and not what the vendor claims. With PoweRec off, if the media is poor quality, your error rate will generally make it unreadable.
If you’re getting garbage recordings, try a good piece of media. The problem is generally with the shoddy quality of the cheap media. You DO get what you pay for.