I guess that it depends on the connection mode of your external HD.
Independantly from spotlight, applications like LR and Bridge seem to be very impacted by slow connections (1 GB ethernet for instance) to external HD.
Signing up for LR as a low level DAM sort of means that you need to invest into at least firewire 800 connection, if not eSATA or better yet SCSI Ultra 320.
Regards,
Bernard