This study focuses on Sudden Ionospheric Disturbances (SID), a collective term for the rapid increase in ionization caused by enhanced X-ray and UV radiation. It is assumed that the SID occurs instantaneously and simultaneously on the entire dayside of the globe. In this work, we question this assumption by analysing 1-second resolution data of ionospheric total electron content (TEC) from worldwide distributed GNSS receivers. We study the global dayside TEC response to thirteen solar flares that occurred between 2003 and 2023, including eleven X-class and two M-class events. For the first time, high-temporal resolution allows to observe a high-speed apparent expansion of SID from the subsolar point toward the dawn/dusk region. We find that this expansion rate has a velocity from ~250-500 km/s, which may depend on the solar flare’s position on the solar disk. The expansion is due to the solar zenith angle impacting the magnitude of the ionospheric response and, consequently, causing the time delay between the TEC increase detected at the subsolar point and in dawn/dusk regions.This expansion is anisotropic i.e., depending on the observation’s location relative to the sub-solar point, and the velocity of the enhancement varies in a different part of the dayside of the ionosphere. This asymmetry can be explained by differences in the background conditions of the ionosphere and thermosphere at the time of the flare, which influence the amplitude of the TEC response and, therefore, the observed expansion velocity. We performed high-rate 10-second Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) simulations that reproduced SID morphology and confirmed the flare enhancement expansion. Our work opens new horizons in the use of high-rate GNSS measurements for studies of the Earth’s ionosphere.