When importing big model files in the STEP format, the memory usage can be higher than expected, thus possibly resulting in a degrade of the system performance.
Most of the said issues can be solved by importing the model entities like surfaces rather than solid entities, which can be done by unchecking the Save/Load solid entities box in the General page of the Open File command options, once you have selected the STEP format.
Please take into account that if you import model entities as surfaces rather than as solids, operations like interference checking or matings will not be enabled. You can easily solve this issue by converting to solids only the very surfaces involved in the operation you intend to perform by using the Make Solid command. The selected surfaces will be converted to skins, that is open solids, that can be processed as real solids though their memory occupation is still very low.