macOS constantly indexes any new files, which can cause mysterious slowdowns.
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.
Read more at MacWorld.com