Avisynth/mpeg2dec slows down Tmpgenc?

[duronator]

I read this article: http://www.doom9.org/mpg/d2a-mpeg2dec.htm and decided to do a little test using Tmpgenc. I tried 3 things:

1. I have an DVD2AVI project file (.d2v) and loaded this directly into Tmpgenc to encode a short clip with the VCD-template.

2. I used VFAPI to make a frameserving AVI-file from the d2v-file and loaded this into Tmpgenc to encode a short clip with the VCD-template.

3. I made a simple avs-file with the following lines:
LoadPlugin("c:\temp\MPEG2DEC.DLL")
mpeg2source("movie.d2v")
and loaded this into Tmpgenc to encode a short clip with the VCD-template.

The first 2 methods were about equally fast. They were both about twice as fast as the avs-method.

My test results seem to disagree with the article above (which states that using Avisynth will increase speed because no YUV2 --> RGB conversion is necessary).

Q1. Why is that?
Q2. Does it make any sense to use an avs-file to encode with Tmpgenc (apart from using specific resize and other filters)?