SMS API Latency and Throughput: What to Measure (2026)

Short answer

Signal House's SMS API responds in under 100 ms, hands messages to the carrier within 500 ms (our standard SLA), and does not throttle accounts. Your only throughput limits are the ones US carriers set for your registered campaign.

When you compare any SMS provider, ask two questions: which part of the journey their latency number measures, and whether they cap your account on top of carrier limits. Most published numbers skip one or both.

The three stages of SMS latency

A text message passes through three stages before it reaches a phone. Providers often quote one number without saying which stage it covers, which makes comparisons misleading.

• Stage: 1. API response · What happens: Your app sends the request and the provider accepts it · Who controls it: The provider
• Stage: 2. Carrier handoff · What happens: The provider checks the message and passes it to the carrier (AT&T, T-Mobile, Verizon) · Who controls it: The provider
• Stage: 3. Carrier delivery · What happens: The carrier filters and delivers the message to the handset · Who controls it: The carrier

A provider can make stages 1 and 2 fast. Nobody controls stage 3: carrier filtering, network congestion, and the recipient's signal all affect it. That is why a provider that promises a fixed end-to-end delivery time is promising something it cannot control.

How to read latency numbers: look at percentiles, not averages. p50 is the typical message, and p99 is the slowest 1 in 100. For login codes and other time-sensitive texts, p99 matters most.

Signal House's numbers

• API response: Under 100 ms
• Carrier handoff: Within 500 ms (standard SLA)
• Account throttling: None. Throughput is set by your carrier-registered capacity, not by a plan tier
• Carrier delivery: Tracked in delivery receipts, not promised, because carriers control it

For reference, Knock's public benchmark measured Twilio's SMS API response at 118 ms median (p50) between July and October 2026. That figure covers stage 1 only, and includes Knock's connection overhead.

The 500 ms handoff covers more of the journey than an API response time does. When you compare handoff numbers across providers, ask what each one includes, such as 10DLC campaign checks and opt-out filtering, because those checks take time and some benchmarks leave them out.

Throughput: carrier limits vs platform limits

Two separate layers decide how fast you can send. Mixing them up is the most common mistake teams make when planning for volume.

Layer 1: carrier limits (apply to every provider)

For US 10DLC, carriers set limits per campaign or per brand, based on your vetting score. Adding more phone numbers does not raise these limits, and carriers treat spreading traffic across extra numbers to dodge them as spam behavior (known as snowshoeing).

• Carrier: AT&T · How the limit works: Messages per minute, per campaign, by message class · Range: 240 to 4,500 SMS per minute for standard campaigns (about 4 to 75 per second)
• Carrier: T-Mobile · How the limit works: Daily cap per brand, shared across its campaigns · Range: 2,000 to 200,000 SMS per day
• Carrier: Verizon · How the limit works: No published rate limits; filters by content · Range: No published limit

Figures from Telnyx's 10DLC rate limits reference. Unregistered 10DLC traffic has been blocked by US carriers since February 1, 2025.

A higher vetting score unlocks a higher limit. See how to maximize 10DLC throughput, or for sends in the hundreds of thousands at once, how short codes give you more throughput.

Layer 2: platform limits (vary by provider)

Some providers add their own account-level caps on top of carrier limits. These are published in their own documentation:

• Twilio: Toll-free sends at 3 messages per second by default, with more through Sales. MMS on long codes is capped at 50 per second per account and "cannot be increased." New accounts without business verification are capped at 20,000 messages per day. (Twilio)
• Telnyx: Account defaults of 50 SMS and 15 MMS per second, with higher limits through Sales. (Telnyx)
• Signal House: No account-level throttling. If your registered campaign supports a rate, that is the rate you send at, with no plan upgrade or support ticket

Why it matters: a platform cap slows you down even when the carrier would allow more. At 50 messages per second, a 100,000-message send takes about 33 minutes. If your campaign is cleared for 75 per second, the same send takes about 22 minutes, but only if your provider does not cap you lower.

Not sure if your current provider caps you? Here is how to tell if your messaging provider is throttling you.

How to benchmark any SMS provider in 5 steps

Published numbers are a starting point. The test that matters is the one you run on your own traffic and registered campaign.

1. Measure the provider and the carrier separately. Time the API response on your side (stage 1). Then send to test phones and record when each message actually arrives (stage 3). If the API is fast but arrival is slow, the delay is carrier-side.
2. Test on all three major carriers. Use a test phone on AT&T, T-Mobile, and Verizon. If one carrier is consistently slower, that points to carrier routing or filtering, not your provider.
3. Test at real volume. Send at the rate your app will actually use, then hold it for 30 to 60 minutes. If response times climb steadily, messages are queuing faster than they are sent out.
4. Record p50, p95, and p99, not averages. An average can hide a slow tail. The p99 is what your slowest users experience.
5. Check delivery receipts. Send to numbers you know are active and numbers you know are inactive, and confirm the receipts report "delivered" and "failed" correctly. Inaccurate receipts make every other number unreliable.

Ask every provider: Do you cap throughput at the account or plan level, on top of carrier limits? What error do I get when I hit the cap? Is raising it automatic, or does it need a sales call?

If you are building an AI agent, speed is conversion: a reply that arrives too late loses the customer. See Building AI Text Agents That Actually Convert for why the send path matters, and how to give your AI agent a phone number to wire one up.

FAQ

What is a good SMS API latency? Compare like with like. For the API response, Knock's public benchmark puts Twilio at 118 ms median, and Signal House responds in under 100 ms. Handing the message to the carrier takes longer: Signal House does it within 500 ms. Delivery to the phone depends on the carrier.

Which SMS API is best for high-volume sending? One that does not cap your account below what carriers allow, and that gets your campaign registered quickly, since carrier limits depend on your registration. Signal House does not throttle accounts, so your registered campaign capacity is your real limit.

Does Twilio throttle SMS? Twilio publishes several platform limits: toll-free numbers send at 3 messages per second by default, long-code MMS is capped at 50 per second per account, and new unverified accounts are capped at 20,000 messages per day.

How many messages per second can I send with 10DLC? It depends on your campaign's vetting score. AT&T allows about 4 to 75 SMS per second per standard campaign, and T-Mobile sets a daily cap per brand from 2,000 to 200,000 messages.

Can I send faster by adding more phone numbers? Not on 10DLC. Limits apply per campaign and brand, not per number, and spreading traffic across extra numbers to get around limits is treated as spam behavior.

Why are my texts delivered slowly if the API is fast? The delay is usually carrier-side: filtering, network congestion, or the recipient's phone being off or out of signal. Compare arrival times across carriers to confirm.

Sending at volume? Signal House does not cap your account, and new accounts get $25 in free credit. Start free.