Standard Load Balancer throttling outbound response throughput to ~25 KB/s for external clients (East US 2)

Question

Our EDS Images Origin service (images-eds.xboxlive.com) behind an Azure Standard Load Balancer is delivering image responses at only ~20-25 KB/s to external clients. The same service achieves 1+ MB/s when accessed directly between VMs within the VNet (bypassing the Load Balancer). This confirms the Load Balancer data path is the bottleneck.

• Images (2-4 MB) take 100-250+ seconds to download through the Load Balancer
• Expected download time: 2-5 seconds
• Observed throughput: ~20-25 KB/s (should be 1+ MB/s)
• Issue is persistent and consistent, not intermittent
• Affects all external clients (CDN, Azure Cloud Shell, direct curl)

Throughput Comparison

| Test Path | Speed | LB Involved? |

|-----------|-------|--------------|

| VM → localhost | 476 KB/s | No |

| VM → another VM directly (within VNet) | 1,134 KB/s | No |

| Azure Cloud Shell → LB → VM | 19 KB/s | Yes |

| External client → LB → VM | 20-25 KB/s | Yes |### 1. Load Balancer Metrics (Feb 4-6, 2026 — PT1M granularity, 3,240 data points)

| Metric | Min | Max | Avg | Assessment |

|--------|-----|-----|-----|------------|

| SNAT Connection Count | 120/min | 199/min | 137.4/min | Stable, no spikes |

| Failed SNAT Connections | 0 | 0 | 0 | No SNAT exhaustion |

| Byte Count | 1,127 KB/min | 22,231 KB/min | 1,582 KB/min (26.4 KB/s avg) | Confirms throttle |

| Packet Count | 1,461/min | 18,309/min | 2,010/min | Normal |

| Data Path Availability | 99.51% | 100% | 99.996% | 130 dips below 100% |

| Health Probe Status | 100% | 100% | 100% | Perfect |