Try a different approach to slow terrain update by always cycling the loop immediately if any data was sent, rather than waiting.

What I believe is happening is that on initial terrain send, this is done one packet at a time.
With WaitOne, the outbound loop has enough time to loop and wait again after the first packet before the second, leading to a slower send.
This approach instead does not wait if a packet was just sent but instead loops again, which appears to lead to a quicker send without losing the cpu benefit of not continually looping when there is no outbound data.

OpenSim/Region/ClientStack/Linden/UDP/LLUDPServer.cs

@@ -1924,7 +1924,8 @@ namespace OpenSim.Region.ClientStack.LindenUDP
                     //
                     // Instead, now wait for data present to be explicitly signalled.  Evidence so far is that with
                     // modern mono it reduces CPU base load since there is no more continuous polling.
-                    m_dataPresentEvent.WaitOne(100);
+                    if (!m_packetSent)
+                        m_dataPresentEvent.WaitOne(100);
 
                     Watchdog.UpdateThread();
                 }