When swapping is enabled, an OS generally has a tendency to reclaim the memory from an application by swapping the data to disk to make more memory available for other programs.

On the Elasticsearch node, this can result in the OS swapping out the heap memory of Elasticsearch. This process of swapping out from memory to disk and then swapping back from disk to memory can slow down the process. This is why swapping should be disabled on the node that is running Elasticsearch.