Older hardware, particularly those with spinning hard drives, benefit from disabling unused services just as with any operating system. On older Macs, Siri can be so slow as to be not that usable anyway.

This change is persistent even after rebooting.

1. click Apple icon in upper left system menu bar and select System Preferences → Siri
2. clear the “Enable Ask Siri” checkbox. The Siri icon also disappears from the top right system menu bar.