We have the same issue since installing sp3, but it only affects certain users of the remote console. I tried the same things as you, but nothing really working to fix the slow write. Once we are in the machine it seems fine, especially with mirror drivers off. Anyone else out there having the same issues?
Our service desk people are reporting the same issue as well. It happens with the mirror driver both enabled and disabled (worse). This was never a problem until 9.0 sp3.
It seems that there is a 5 second delay when typing or maybe a screen refresh problem.
The behavior is not really consistent. I can remote to a computer that had this issue from my workstation and it'll work fine for me, even though 10 seconds before someone else was remoted in and was having this issue. But i've also experienced this from my console.