The speed of Siri's speech can be changed at Settings / Accessibility / Siri. Non-VoiceOver users can drag the slider between values of 80% and 200% with 100% being the default speed. VoiceOver users can change the speed of Siri's voice by selecting the speaking rate slider and then swiping up or down to adjust the speed in increments of 10%.
Test your setting by asking Siri something with a reasonably short answer such as "when is sunset?".
Fine adjustment for VoiceOver users is possible by double tapping and resting your finger on the screen, waiting for the rising tones and then sliding your finger slowly left or right. You may need several attempts to get this fine adjustment gesture to work.