Nightly backup crawls at 8.7 Mbit/s over an idle 1 Gbit/s circuit

The nightly backup from SRV1 (10.1.10.20) in DC1 to BKP1 (10.2.20.30) at the DR site stopped finishing before morning about a month ago. It runs over a dedicated 1 Gbit/s circuit between R1 and R2. The backup tool reports a steady 8 to 9 Mbit/s for the whole run. The circuit graphs show it almost idle, interface counters on R1 and R2 are clean, and the carrier has tested the circuit at full rate. The server team says nothing changed on either server.