  We study the critical slowing down towards the continuum limit of lattice QCD
simulations with Hybrid Monte Carlo type algorithms. In particular for the
squared topological charge we find it to be very severe with an effective
dynamical critical exponent of about 5 in pure gauge theory. We also consider
Wilson loops which we can demonstrate to decouple from the modes which slow
down the topological charge. Quenched observables are studied and a comparison
to simulations of full QCD is made. In order to deal with the slow modes in the
simulation, we propose a method to incorporate the information from slow
observables into the error analysis of physical observables and arrive at safer
error estimates.
