Thermodynamic variables exert a strong influence on the flow field in high-speed combustion. Direct simulation of the shear layer is used to investigate the coupling between thermodynamic fluctuations and the turbulent flow in subsonic-to-supersonic regimes and over a wide range of density ratios. The high-speed effect is parametrized by the convective Mach number, Mc, while density heterogeneity is characterized by the ratio of free-stream densities, s = ρ2/ρ1 With increasing Mc, a large reduction in thickness growth rate is observed. Analysis and direct simulation is used to offer a mechanism based on the finite speed of pressure fluctuations and consequent temporal decorrelation to explain the reduced growth rate in the high-speed regime. The character of the underlying thermodynamic fluctuations is found to change from the uniform density case to the non-uniform cases. The acoustic mode dominates the former while the entropy mode dominates the latter. An increase of the density fluctuations, with increasing s, is observed while pressure fluctuations decrease.