Short answer
If your 20k leads/month are evenly spread and each lead triggers only 1–2 simple Zap actions, Zapier can work short-term. If you expect bursts, multi-step processing, low per-lead cost, strict SLAs, or complex error handling, build a custom webhook/queue service.

Recommendation
Start with Zapier for a 30–90 day pilot to validate flows and integrations, then move to a custom webhook + queue + workers architecture if cost or reliability metrics are unsatisfactory. A hybrid approach (Zapier for low-volume or non-critical integrations, custom for the high-volume core path) is often optimal.

Why: cost, rate limits, retries (quick comparison)
- Zapier: fast to implement, wide integration coverage (good when you need ready-made connectors). Zapier counts “tasks” (each action) toward your plan. Webhooks (Catch Hook) are instant, but Zapier enforces concurrency/throughput per account and per connected app; Zapier also has retry/backoff but limited visibility and limited retry depth for some errors. Cost scales with tasks — if you have multiple actions per lead the monthly bill grows rapidly.
- Custom webhook service: higher upfront dev + ops cost, but you control concurrency, batching, retry policy, idempotency, backpressure, and can optimize per-lead cost. You can integrate a durable queue (SQS/Google Pub/Sub/Kafka), autoscale workers, and implement exponential retries + dead-letter and monitoring.

Decision criteria (use these to choose)
- Burstiness: if you have spikes (thousands of leads in minutes) → custom or queue in front of Zapier.
- Actions-per-lead: >2 actions/lead favors custom due to task multiply effect.
- SLA & reliability: strict SLAs, guaranteed retries, observability → custom.
- Budget and time-to-market: limited engineering bandwidth/budget → Zapier first.
- Long-term volume: >50k leads/month almost always favors custom for cost control.
- Team: small team with no SRE → Zapier; dev+ops team available → custom.

Practical checklist to decide and/or implement
1) Measure traffic: instrument current form/ingest to capture per-minute peaks and avg leads/day.
2) Count tasks: leads × actions-per-lead = monthly Zapier tasks. Compare against Zapier plan limits and task add-on pricing. (Also account for retries and reprocessed failures.)
3) Pilot: wire 1–2 critical flows to Zapier for 30 days; log failures, latency, and cost.
4) If moving custom: design webhook → queue → worker model; implement idempotency keys, DLQ, metrics/alerting, and backpressure (rate limiter or batcher) to downstream APIs.
5) Consider hybrid: buffer/CMS writes via your webhook/queue and then fan-out to Zapier or other services for non-time-critical work.
6) Load test with realistic bursts before cutover.

Best-for / Avoid-if
- Best-for Zapier: fast proofs-of-concept, many different SaaS connectors, small teams, non-critical low-volume flows.
- Avoid Zapier if: sustained high volume, tight cost targets, complex retry semantics, or heavy burst traffic.

When the answer depends
Budget, team size/skill, and whether this is a short-term MVP vs. long-term product will determine the right choice. If you want, I can help estimate monthly task cost from your real actions-per-lead and peak rate and draft a simple queue-worker architecture for migration.

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