Siri isn't that slow for me generally, but the responses are relatively primitive compared to GPT. Very likely Siri is lagging GPT based agents due to the high processing cost of LLM queries.
Startups can launch with today's GPT agents because their utilization is relatively tiny. Siri operates at planet scale, plus Apple is committed to on-device privacy, so they need to develop highly optimized on-device models.
No doubt this is going to drive a major hardware upgrade cycle for Apple.