In my opinion, to answer this question, a term called "quality factor" has to be mentioned here. Quality factor represents the spectral energy density of the LSPR mode, which can be derived from the expression: Q = λres/Δλ, where Δλ is the FWHM and λres is the resonant wavelength, respectively, obtained from the absorption spectra. Au and Ag have narrow resonant band, and therefore high quality factor and large spectral energy density. This is because Au and Ag have large -real (eps)/ imag (eps) ratio. You can refer to this paper for the details:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.746.2583&rep=rep1&type=pdf