After upgrading to Big Sur from Catalina, I remarked my fan was running continuously. Opening Activity Monitor I remarked that "com.apple.SpeechRecognitionCore.speechrecognitiond" was chewing through CPU at 37.3%.
Looking a likely candidate, and just being a bit anal about having a microphone in an always-on state, I wanted to knock that nonsense on the head.
There was a microphone graphic on my screen which presented the option to "Sleep" which when clicked then presented the option to toggle the mic to "Wake Up". Clicking this did nothing: Speech Recognition nonetheless kept the fan humming and chomping through CPU cycles.
After toggling every application's access to the microphone to deny access, "com.apple.SpeechRecognitionCore.speechrecognitiond" was still happily chomping through CPU.
Where is a setting to toggle the microphone off at OS level in Big Sur?!?!?!