It has been shown, for example in the papers
- Deng, Du, The Computation of Approximate Competitive Equilibrium is PPAD-hard
- Hirsch, Papadimitriou, Vavasis, Exponential Lower Bounds for Finding Brouwer Fixed Points
that finding competitive equilibria is in a certain sense computationally intractable. So how does economic theory expect a market to reach a competitive equilibrium if each agent is presumably representable by an efficient computational process? In fact, parallel computation doesn't really help because a polynomial number of processors can be simulated in polynomial time.