How windows handle the clipboard interface with Xming?
Your observation that the delay is proportional to the number of characters pasted should be expected, since each of those characters must be fed through the SSH terminal, a serial pipeline. Additionally, rendering those characters on your end takes some effort from Windows. I suspect that the reason that you see less delay with your VBScript paste operation is that the VBScript paste operation largely eliminates the user interface from the process, since the clipboard can deal with the characters, without having to figure out how to draw them.