  Due to their cosmological distances high-energy astrophysical sources allow
for unprecedented tests of fundamental physics. Gamma-ray bursts (GRBs)
comprise among the most sensitive laboratories for exploring the violation of
the central physics principle of Lorentz invariance (LIV), by exploiting
spectral time lag of arriving photons. It has been believed that GRB spectral
lags are inherently related with their luminosities, and intrinsic source
contributions, which remain poorly understood, could significantly impact the
LIV results. Using a combined sample of 49 long and short GRBs observed by the
Swift telescope, we perform a stacked spectral lag search for LIV effects. We
set novel limits on LIV, including limits on quadratic effects, and
systematically explore for the first time the impacts of the intrinsic GRB
lag-luminosity relation. We find that source contributions can strongly impact
resulting LIV tests, modifying their limits by up to a factor of few. We
discuss constraints coming from GRB 221009A.
