We consider the uplink of a Massive MIMO network with L cells, each comprising a BS with M antennas and K single-antenna user equipments. Recently,  studied the asymptotic spectral efficiency of such networks with optimal multicell minimum mean-squared error (M-MMSE) processing when M and K is kept fixed. Remarkably,  proved that, for practical channels with spatial correlation, the spectral efficiency grows unboundedly, even with pilot contamination. In this paper, we extend the analysis from  to the alternative regime in which M, K with a given ratio. Tools from random matrix theory are used to compute low-complexity approximations which are proved to be asymptotically tight, but accurate for realistic system dimensions, as shown by simulations.