Running on media the latency is low,on hd is high.

If I run Debian 7 + Linuxcnc the cd and leaving open with several things, including mp3 and video, stabilized 17385ns.
  But if I run hd ide 40gb, easy passes of 500000ns running the same things and using the same driver (nouveau).
Motherboard ASUS K8V-X SE + Sempron 2600+
2gb ram
Video Geforce 5200
HD40GB IDE
I noticed something else, I made the boot from the hd in recovery mode and the latency is just 14500ns even me doing a lot of things at once, but does not mount more the stick does not work the network / internet.

  Any suggestions of what I can do to try to solve?

Found the problem, if you use both motherboard IDE ports, the latency is too high, using the ide dvd drive and HD on the same flat cable, the latency is at most the 15000ns.

Just FYI, I use an ASUS board from about the same era (see here ), and I replaced the old IDE disk by a cheap SATA SSD.
I disabled the IDE interface in the BIOS (used an USB CD drive for the install) and latency was better after that.

Note that the BIOS is important as well, I upgraded to the latest one and latency was really bad, so I reverted to what I had before (I did not tried all the version though )

I understand, in the case here I installed linuxcnc / debian in HD and then hung up the drive CD / DVD and the latency was between 15000 and 17000, as I will not need to use the drive dvd / cd there can let him off. With the release of the old bios or the latest the same thing happens.
Thanks for the answer.