I came to know that the PACS instrument of HERSHEL has a resolution of 5" at a scan speed of 20''/sec whereas it's 10'' at 60''/sec. The resolution has almost doubled with the scan speed. Can you please tell me why it's so?
See Table 3.1 in that link. Are you referring to the doubling of the FWHM from 5.26 x 5.61 at 10"/s to 5.75 x 9.0 at 60"/s with the Blue detector? If so then that's only along the spacecraft z-direction, which, as the mention above, is due to the increased scan speed. PACS is a bolometer so it doesn't bin photons, it reads a change in temperature due to those photons as a signal instead. When the spacecraft scans faster the thermal properties of the detector mean it can't respond as fast so you smear the temperature-based signal through time, which translates into spatial resolution. Also that signal comes out as a time series, which must be sampled, so I assume that's what they mean when they refer to data averaging.
I don't have a lot of experience directly with Herschel but I've worked with other bolometric imaging systems so this is just my take on their manual.
The decrease in resolution (softer) is analogous to the effect of lag in old analogue TV cameras. A moving object will be 'smeared out' because of the delay in building up the image on the camera and the rate the object travels across a given spot. The image on the back of the tube is not over the sensor element for long enough to register fully and the charge on the sensor takes time to decay so it leaves behind a trace of the object that passed. A stationary object will give a full resolution image and the resolution will get worse as the speed increases and more picture elements contain traces of the image moving across them.
The acceptable angular speed will be inversely proportional to the decay time of the image on the sensor.
It is due to the Convolution of the time sensitivity function and the time profile of the light (heat) landing on a spot. (Those of a nervous disposition can ignore this bit.)