Nero will burn at whatever the burn & media combination will allow.
If 2.4x is all that's offered then the burn's firmware does not recognise the media (DVD) you're using & defaults to 2.4x.
Use Nero's CD-DVD Speed , disc info option - to get the media details & the firmware being used.
Save this picture using the floppy disc at the top right & save as a PNG file. Then post the picture back here using the Manage Attachments option.
This is what you see.