On any Mac, you can experience odd slowdowns when copying or acting on a large number of files, such as duplicating a folder with many items in it, cloning a drive, or testing drive performance. That’s because Spotlight never sleeps. It’s constantly looking for modified or new files to index.
Spotlight’s “polling” can have a significant performance toll. For example, cloning speed might be half the rate you expect or even lower. But you can tell Spotlight to keep its hands off while performing operations that it might otherwise slow down. The secret is the Spotlight preference pane’s Privacy view (System Preferences > Spotlight > Privacy).
Apple uses the term “privacy” to indicate you don’t want items indexed that are shown in the volumes or folders list in that view. But it’s really a “don’t index me” list.
You can use the Privacy list in a lot of ways:
Add external volumes that contain backups, like networked Time Machine volumes or local clones. These don’t need to be indexed. (Volumes that are entirely devoted to Time Machine are already excluded, but don’t show up in the list.)
When creating or updating a clone, drag its volume icon to the list before starting. You can remove it later, but you likely don’t need the clone indexed within the current startup volume’s system.
Create or add folders that commonly hold large numbers of temporary files or are used for rapid reading and writing of data that doesn’t need to be indexed. I discovered in using the Blackmagic Disk Speed Test, a free app for determining drive performance, that placing its test file in a Spotlight-indexed folder dramatically reduced tested throughput compared to a folder excluded from Spotlight.
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