I encountered this exact issue and resolved it. The 10-second delay is caused by IPv6 connectivity being blocked in your Security Groups.

My Setup

• Service A: Fargate task with YARP reverse proxy + Service Connect

• Service B: Fargate task with REST API

• Configuration: Using Service Connect service discovery hostname

• VPC: Dual-stack mode enabled

The Fix

**Add IPv6 inbound rules to your Security Groups

This is root cause as per Claude AI:**

When using ECS Service Connect with Fargate in a dual-stack VPC:

1. Service Connect attempts IPv6 connection first (standard .NET/Java behavior per RFC 6724)

2. Security Group silently drops IPv6 packets (if IPv6 rules aren't configured)

3. TCP connection times out after exactly 10 seconds (default SYN timeout)

4. System falls back to IPv4, which succeeds immediately