SharePoint 2013: search crawls taking unusually long time to complete

Problem

You find that crawls are taking unusually long on your production farm, with craw rates around 7-10 dps, where 30-40 is what should be expected.  You check user rights assignments of the service account and find that all user rights are appropriately configured.  You also find that your staging farm, which is configured identically to your production farm, shows expected crawl rates of 35+ dps.  You are unable to immediately identify what is different between the two farm configurations.  

Solution

• Check the Enterprise Search Service performance level using Get-SPEnterpriseSearchService.  If this is Reduced or Partially Reduced, change to Maximum.

Reference

• Primary
• SharePoint 2013: Service Account Configurations and Permissions
• Set-SPEnterpriseSearchService
• Get-SPEnterpriseSearchService

Notes

• It had been over six months since I last engaged this configuration item and so had quite forgotten it.  This was resolved with the help of Microsoft Support.
• Initially, experienced full crawl times of about 4 hrs on 110K items at 65 GB, with crawl rate of 7-10 dps.  Then discovered performance level was set to Reduced.  Changing this to Partially Reduced improved these metrics to full crawl time of about 1 hr, with crawl rates 40-42 dps.  This is a 400% improvement in going from Reduced to Partially Reduced.