As you can see the screenshot below, why aggregate multichannel sampling rate is lower than single channel sampling rate?
You have a device that is muxed. The A/D conversion is shared between all of the channels. You would need a card that has a separate A/D for each channel, like the 6366.
You didn't get what I mean. Even though sampling rate is shared by channels, but in principle the aggregate rate should be same as maximum single channel rate.
I searched similar question, seems reason is channel needs switching time for multichannels which sacrifice half sampling rate.