While preprocessing, ILOG CPLEX attempts to strengthen bounds on variables. This bound strengthening may take a long time. In such cases, you may want to turn off bound strengthening.

ILOG CPLEX attempts to reduce coefficients of constraints during preprocessing. Coefficient reduction usually strengthens the continuous relaxation and reduces the number of nodes in the branch & cut tree, but not always. Sometimes, it increases the amount of time needed to solve the linear relaxations at each node, possibly enough time to offset the benefit of fewer nodes. Two levels of coefficient reduction are available, so it is worthwhile to experiment with these preprocessing options to see whether they are beneficial to your problem.

The RelaxPreInd parameter controls whether an additional round of presolve is applied before ILOG CPLEX solves the continuous subproblem at the root relaxation. Often the root relaxation is the single most time-consuming subproblem solved during branch-and-cut. Certain additional presolve reductions are possible when MIP restrictions are not present, and on difficult models this extra step will often pay off in faster root-solve times. Even when there is no appreciable benefit, there is usually no harm either. However, the RelaxPreInd parameter is available if you want to explore whether skipping the additional presolve step will improve overall solution speed, for example, if you are solving a long sequence of very easy models and need maximum speed on each one.

ILOG CPLEX preprocesses a MIP by default. However, if you use a basis to start LP optimization of the root relaxation, ILOG CPLEX will proceed with that starting basis without preprocessing it. Frequently the strategic benefits of MIP presolve outweigh the tactical advantage of using a starting basis for the root node, so use caution when considering the advantages of a starting basis.