IK constraints are extremely unlikely to cause your performance problems, and 5 IK constraint calculations in a frame will not cost you any number of cycles.
What is more likely is that your skeleton has to many meshes with to many vertices. Can you give us some statistics about your skeletons? How many do you render? How many meshes/vertices do they consist of? You can use the metrics view to get some stats on your skeleton. Metrics - Spine User Guide