This message is triggered in case you have either disabled “Max. sample intensity”, or set it’s value too high. In both cases, such setup can result in significantly worse performance or fireflies that will clear up extremely slowly.
Max. sample intensity is disabled (set to 0)
Max. sample intensity is set to value greater than 100
Make sure that Max. sample intensity is set to some reasonable value, ideally default value of 20, and make sure the entered number is not too high, nor set to 0.
See: What is Max Sample Intensity (MSI)?