Depending on your hardware if your crawling your own content, or if you are crawling external content you might like to know what the three settings for the crawler performance settings really mean.
I ran across this article Change the Indexer Performance Settings which provides the following table:
| Indexer Performance setting | Total threads available for crawling | Maximum threads available for crawling any particular host |
| Reduced | Number of processors on the index server | Number of processors on the index server |
| Partially reduced | 4 times the number of processors on the index server | Number of processors on the index server plus 4 |
| Maximum | 16 times the number of processors on the index server | Number of processors on the index server plus 4 |