I realized the other day that on OS X, the Spotlight indexing process is
started using launchd. This makes it very easy to modify the launchd
configuration script to insure that background indexing uses the least amount
of CPU and I/O bandwidth possible.
Edit the configuration script by running this command as root:
# open /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
You should find yourself in the Property List Editor application. Now add two keys at the top-level, one named LowPriorityIO, which is a boolean set to true, and another named Nice which should be an integer set to 20.
Now whenever the mds spawns mdworker processes to index recent changes to the
file system, it won’t get in your way quite as much as before. (Without this
change, mdworker processes run at the same priority as user processes,
according to output from the ps axl command).