Preconditioners are used by iterative solvers to improve the convergence speed of the solving process. Increase in convergence speed is related to the number of iterations necessary to get a converged solution. So while in general the use of a preconditioner means that the iterative solver will perform fewer iterations it does not guarantee that the actual solution time decreases given that some preconditioners can be expensive to setup and run.

Note that in general changes to the matrix will invalidate the preconditioner if the changes occur after creating the preconditioner.