Crawl Rules vs Crawler Impact Rules
SharePoint Search has two similar sounding configurable options that are easy to confuse.
They do two very different jobs.
Crawl Rules allows you configure how SharePoint crawls within a Content Source.
You use them to tell SharePoint what, and as whom to crawl.
For instance “when you crawl this path, use these credentials” …or “exclude this path from crawling.” Crawler Impact Rules tells SharePoint how hard to crawl a path within a Content Source.
For example, if you’ve set up SharePoint to index itself efficiently, you can create a rule that states: “crawl this path as fast as you can, with up to 8 documents at a time”
However, if you’re trying to include in your index content from a rickety old SharePoint farm in your shiny, new one, you might want to create a more limiting Crawler Impact Rule:
e.g. “crawl this path with at most one document at a time, and leave 60 seconds between each one.”