Too slow real speed when burning DVD

Hi, everyone

I have a problem with my Combo device (TSSTcorp) on my Dell Inspiron. When I burn a DVD data disk with Nero it is burned at 1X to 1.2X. It doesn't matter which speed I have set (4X or 8X). So it takes approximately 50 minutes to burn a full DVD and after that the verification takes same time at this speed again.
When I burn a CD the real speed is again slower than the speed I have chosen. And at the end of the burning process I get a message saying that "buffer underrun error has been avoided hundreds of times" or something like this.

So, where is the problem? Is it a hardware problem?

 

Either the computer is running too slow, the source of the data (HDD or other) is too slow, or the buffer isn't large enough.

 

I got an improvement but I don't now exactly why.

I reinstalled Windows, changed the Nero version, updated BIOS and device firmware, set the SATA drive to use AHCI and set the same drive (via BIOS Setup) to be noisier but with more performance.

And the result:
Burning a DVD-R with 3-4X - average speed calculated by me - 3.8 X (the desired speed is the maximum drive speed - 8X, the maximum media speed is 16X)
Burning a CD-R with 9-22X (increasingly) - average speed calculated by me - 13.5 X (the desired speed is the maximum drive speed - 24X, the maximum media speed is 52X)

Other differences:
No more "buffer underrun error"
The buffer level shown by Nero is ~98% during the whole burning! It was different before! It used to change during the whole burning and used to get empty often.

Should I be glad with this speeds? I'm asking because the laptop is in the warranty term but I don't now if this problem can be admitted as a device problem.