Measurement Speed Consideration - DATATEC EDU34450A Guia Del Usuario

The best way to eliminate ground loops is to isolate the instrument from earth by not grounding the input terminals.
If the instrument must be earth-referenced, connect it and the device-under-test to the same common ground
point. Also connect the instrument and the device-under-test to the same electrical outlet whenever possible.

Measurement Speed Consideration

There are two methods of integrating the sampled data taken in the measurement, slow/medium (NPLC) and fast
(Aperture).
When you set the resolution to slow or medium, you can achieve improved accuracy associated with time averaging,
as well as also improve the rejection of power-line interference (normal mode rejection, or NMR).
Aperture is the period, measured in seconds, during which the instrument's analog-to-digital (A/D) converter
samples the input signal for a measurement. A longer aperture yields better resolution; a shorter aperture provides
faster measurements. For example, the fast mode for DC Voltage measurements set a specific 1 ms measurement
period and is not based on power line frequency. No normal mode rejection is provided in aperture mode.
Keysight EDU34450A User's Guide 101