In many posts ,I read that local pressure or pressure difference is considered by the corresponding post’s Author to be conductive to a IAS /TAS increase or decrease. This QNH local differences must be ‘high’ to justify such fluctuation. However, please all keep in mind that a 1 mb / 1 hPa difference of pressure over , let’s say’ 50 km will result in winds of gale force (well , sort of), not suitable for air tests in stabilized conditions.
The same applies for windshear: it is so strong to have the anemometer significantly fluctuating by 10 KIAS , it must be of short duration(so no value for a stabilized flight) or the aircraft is for sure in/close to a thunderstorm, and also this condition is clearly not suitable for any performance speed test. If, instead , the windshear is not notable on the aircraft (no bumps, etc) in my opinion is ininfluential over the 5-10 minutes interval necessary for a stabilized measurement.
Daniel