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
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