From datasheet, 4070 can read up to 3kS/s using 5 1/2 bits. However, I can only read up to about 18 samples per second under NI Signal Express by choosing "run as fast as possible". Why is my device reading so slowly? How could I rise the sampling speed? If I use labview function, it is still quite low.
The 3kS/s is when in digitizing mode. You are currently just telling it to measure single samples whenever the software gets around to asking it for data. 18S/s is pretty good when software timed.
In LabVIEW, look at the VIs in the NI-DMM->configuration palette. There is a VI in there for configuring the "multipoint" acquisition and more in the Acquisition palette for you to play with.
The 3kS/s is using multipoint mode with no switch involved, not digitizer mode. In digitizer mode you can get up to 1.8MS/s. There is an example called Maximizing DC Reading Rate that you can use to try different timing scenarios and see what your reading rate and noise free resulution are for your specific setup. Be aware that the effective noise free resolution reading is not only determined by the speed of the DMM but noise in the source signal as well as any noise caused or picked up by cabling.