On‑premises ISP is a Partner Interconnect provider, on‑prem VPN uplink 10 Gbps, VPN throughput limited to 500 Mbps by packet loss, GCP→on‑prem traffic dominant, occasional bursts to 1.5 Gbps, minimize cost and complexity. Which connectivity to provision?

Provisioning a Partner Interconnect through your ISP is the most suitable option. Since your on-premises ISP is a Partner Interconnect provider, this leverages existing infrastructure and relationships, minimizing complexity. Partner Interconnect offers higher bandwidth options (up to 10 Gbps per VLAN attachment) and lower latency compared to VPN, addressing the current 500 Mbps VPN throughput limitation and accommodating bursts up to 1.5 Gbps. This solution is generally more cost-effective than Dedicated Interconnect for these bandwidth requirements and avoids the operational overhead of managing multiple VPN tunnels. Dedicated Interconnect is typically for much higher bandwidth needs (10 Gbps or 100 Gbps ports). Multiple VPN tunnels with ECMP can increase bandwidth but add complexity and still have VPN overhead. VPN compression is not a reliable or significant solution for increasing throughput to 1.5 Gbps.