Playback of video is slow in GT520 than ION2010

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We evaluate the video playback in mplayer with the Video Acceleration feature(vaapi) in the ZOTAC D2700-ITX.
But ZOTAC D2700-ITX is not smooth playback of video compared to ASUS AT5IONT-I.

There are two levels of Level-1 and Level-0 at
[GPU0 (GeForce GT 520) - PowerMizer - Performace Levels] - [Performace Level]
in Nvidia X Server Settings.

[Performace Level] change to Level-1 from Level-0 when you start the video playback in mplayer.
But, [Performace Level] will return to Level-0 from Level-1 after about “25” seconds.

I changed to [Prefer Maximum Performance] from [Adaptive] in [PowerMizer Settings-Preferrd Mode] of [Nvidia X Server Settings].
Because I would like to evaluate the Level-1 of [Performace Level].
But video playback in mplayer by ZOTAC D2700-ITX is not smooth compared to the ASUS AT5IONT-I.

The evaluation of video playback was done by swapping the same SSD.
So software environment is exactly the same.
The ZOTAC GPU is the GT520 and the ASUS GPU is ION2010.
The performace of ZOTAC GPU/CPU is greater than the ASUS GPU/CPU.
The following is an environment ASUS AT5IONT-I and ZOTAC D2700-ITX.

CPU : Intel Atom D2700 2.13GHz : Intel Atom D525 1.8GHz
GPU : Nvidia GeForce GT520 : Nvidia ION2 ION2010
CUDA Cores : 48 : 16
VBIOS Version : 75.19.1b.00.b2 : 70.18.5d.00.1d
VRAM : 512MB : 512MB
Graphics Clock: 270 MHz/810 MHz : 135 MHz/405 MHz/535 MHz
Memory Clock : 405 MHz/897 MHz : 135 MHz/405 MHz/790 MHz
Processor Clock : 540 MHz/1620 MHz : 535 MHz/790 MHz/1230 MHz
Memory : DDR3-1066 2GB : same as left
SSD : OCZ Vertex 4 SATA III 64GB : same as left
OS : Ubuntu 10.04LTS : same as left
Linux kernel : 2.6.32-44-generic : same as left
Nvidia Driver : 304.60 and 304.84 : same as left
libva : 0.32.0-1+sds2 : same as left
libvdpau1 : 0.3-2build1 : same as left
vdpau-video : 0.7.3-1 : same as left
mplayer-vaapi : SVN-r35107-4.4.3 : same as left

Please let me know the answers to the following questions and how playback of video in mplayer becomes smooth at ZOTAC D2700-ITX.

  1. Why does [Performace Level] go back to Level-0 from the Level-1 after about 25 seconds while playing back videoe?

    ASUS AT5IONT-I is always Level-2 while playing back videoe.

  2. Why can’t play back video smoothly even if the Level-1 of [Performance Level]?