How can a very low species mass diffusivity affect the residual convergence?
A very low interspecies mass diffusivity can affect the convergence specially in regions with low turbulent diffusivity (viscosity). The diffusive part of the species transport equation can become extremely small compared to the convective part and thus might create problems with convergence.
A workaround can be to start with a higher laminar mass diffusivity for the species and then slowly drop it.