Possibilities for optimizing the work-horse of phase equilibrium calculations, the two-phase PT-flash have been investigated. Significant improvements relative to conventional approaches are obtained by using a full Newton's method for solving the equilibrium equations and by combining the evaluation of thermodynamic properties for the two phases into a single evaluation of the quantities of interest. The algorithm is tested on a compositional simulation where it is shown that for a hydrocarbon mixture with a number of components that enables a realistic description of a reservoir oil (10-15) it is possible to carry out about 5000 rigorous flash calculations per second on a personal computer.