Why does this setting cause 503 on Analytics API at load?

What is the reason this setting causes 503 Service Unavailable when I hit /api/v2/analytics/conversations/details/get with JMeter 5.6.2 from Singapore? Single requests return data fine, but at 20 concurrent threads the response time spikes to 30s before failing. The timeframe is set to now-15m and group_by is participant.id.