API Latency Comparison: Which ESP Responds Fastest?
API latency is the unsung hero of email infrastructure performance. While delivery speed captures headlines, the time it takes for an ESP's API to acknowledge your send request directly impacts your application's responsiveness and user experience. Our instrumented testing across six geographic regions reveals surprising variance in this critical metric.
Brew leads the field with a median API latency of just 28ms, processing requests nearly instantaneously from the application's perspective. This performance stems from their edge-deployed API infrastructure, which routes requests to the nearest node before propagating to their core sending system. Resend follows at 32ms median, while Postmark rounds out the top three at 35ms. These sub-50ms response times mean email sending adds imperceptible latency to user-facing operations.
The gap widens considerably as we move down the rankings. SendGrid averages 65ms with a P95 of 140ms, which begins to introduce noticeable delays in high-throughput scenarios. Amazon SES, despite its massive infrastructure, posts 72ms average latency, likely due to the additional overhead of AWS's authentication and routing layers. Marketing-oriented platforms like Mailchimp show P99 latencies exceeding 350ms, making them unsuitable for latency-sensitive transactional use cases.
Geographic analysis reveals an interesting pattern. US-centric providers tend to show 30-50% higher latencies for requests originating from Europe or Asia-Pacific regions. Brew and Postmark have invested in multi-region API deployments that maintain sub-50ms latencies globally, while others show marked degradation. For applications serving international users, this geographic consistency may outweigh raw performance numbers.
Our recommendation for latency-sensitive applications is clear: prioritize providers with documented P95 latencies under 100ms and verify performance from your primary deployment regions. The difference between a 30ms and a 200ms API call may seem academic until you're processing thousands of transactional emails per minute and watching your request queue grow.