High speed dubbing of digital information is not the same as that of analogue (audio cassettes) where there was a loss of high frequencies due to the increased speed of the tape over the recording heads.
What is happening in high speed HDD to DVD dubbing is transfer of data rather than pictures and sound. The data exists as a string of 1s or 0s this means that if the DVD drive is capable of it you can transfer the data at faster than normal speed. There is no loss of quality as you aren't in losing any data.
The only time you will not have the ability to dub at high speed is if the recording on the HDD is at a different quality setting than the DVD. For example you recorded a film at standard quality (2hrs per DVD) which lasts 2hrs 30 mins. If you wanted to then burn this onto DVD the machine would need to transcode the recording to make it fit onto the DVD. Most machines do this in real time so would take 2hrs 30 mins to do and then you would need to finalise it.
Most DVD recorders have a quality setting and most default to 2 hrs per DVD as the standard quality. Some allow higher quality (1hr or 1.5 hr ) but on most domestic equipment you would not really see any difference on broadcast pictures.
The DVD recorder will also allow you to fit more than 2 hrs on to a DVD by reducing the data rate of the recording giving you 3, 4, or 6 hrs on a single DVD. This is where you will begin to see the difference in picture quality as artefacts (blockiness in the picture detail) and jekiness in fast motion stuff. You will see this esp. if you slow mo through fast paced action. I hope this answers your question.