Small, irregular-shaped mesh elements, such as those that describe model details like bolt holes, or small fillets, result in small time steps. If you can eliminate unnecessary details in your geometry, such as small-radius fillets or holes, you will have fewer very small elements in the model and the time step size will increase accordingly.
Examine your model carefully to see where you can eliminate geometric details. Ask yourself, do I really need to model that bolt hole, or, do I really need that tiny fillet? Removing these kinds of geometric details can result in dramatic reductions in run-time.
Determine whether you can take advantage of symmetry to model half you model or less. With these simplifications, improvements of an order of magnitude or more are easily accomplished.