Two projects in different organizations run in separate VPCs with non-overlapping subnets. Ensure all instances communicate over internal IPs with minimal latency and maximum throughput.

VPC Network Peering allows direct, internal IP connectivity between two VPC networks, even if they belong to different organizations, without using external IP addresses. This provides low latency and high throughput, fulfilling the requirements. Cloud VPN creates an encrypted tunnel over the public internet, which introduces higher latency and lower throughput compared to peering. IAP TCP forwarding is designed for secure SSH/RDP access to instances, not for high-throughput, low-latency internal communication between applications. Building an OpenVPN tunnel between instances is a manual, software-defined solution that adds overhead, management complexity, and doesn't offer the native performance of VPC peering.