I checked the document you have posted and I looked for more information in order to find if someone else has done more tests with this and if someone has been able to use a higher sample rate.
I found the following:
The ADC has a 20 MHz clock rate and the Sample Rate of it is 1MSPS. There are other threads where you can find more discussions related to this:
This one has very useful information: Maximum ADC sampling rate.:
...the ADS7951 can be connected to 1Mhz SPI to sample and transfer data at 1 MHz. Edison SPI linux driver in the current release that is used by Arduino for Edison can only reach 2.9KHz. Intel made the SPI driver source code available through the BSP for the users to make changes and customize the SPI driver to fits their needs.