Diagnose very slow Remmina session for VirtualBox VM

I'm using Remmina 0.9.99.1 with CrunchBang VMs on a remote server (VirtualBox 4.3.4 on Debian 7.2.0 x64). I'm using RDP with TLS security, and connect through a UDP-mode VPN. With freshly-installed CrunchBang VMs, Remmina sessions are unusable, with high latency and retransmission artifacts. The VPN becomes saturated, and ping increases from 200-300 ms to 2-4 seconds. However, Remmina sessions become usable after setting "xset r off" in OpenBox autostart to disable keyboard repeat. What might be causing this? In addressing questions about similar problems, some have recommended the disabling of receive window auto-tuning. But I see nothing relevant in documentation for either VirtualBox or Remmina. Given that I'm using a UDP-mode OpenVPN connection, I don't believe that this is a classic TCP-via-TCP retransmission storm.