The presence of a gas volume-density gradient inside star-forming regions allow them to raise their star formation rate (SFR) compared to what they would experience if their gas was of uniform density. I define the “magnification factor 𝝵” as the ratio between the SFR of a centrally-concentrated clump and the SFR that this clump would experience should its gas be uniformly distributed. I show that magnification factors higher than 10 are achieved by power-law gas density profiles with logarithmic slopes steeper than -3. Such steep density profiles describe well the densest regions of the nearby molecular clouds MonR2 and NGC 6334. Clumps with a high magnification factor form stars much faster than expected based on the mean free-fall time of their gas. Therefore, not only does a gas density gradient inflate the clump SFR, it also inflates the star formation efficiency per free-fall time that we measure. The diversity in measured star formation efficiencies per free-fall time thus partly reflects the diversity in star-forming region structures. I provide a method to quantify the contribution of the gas density gradient of a clump to its SFR, thus allowing one to estimate its magnification factor.