Hello WM1986 ,
Thank you for your post. SolveigMM Video Editing Engine to be used in WMP plugin tries to guarantee video/audio synchro + validity of an output AVI file. Therefore it reads AVI files from begining wherever selected fragment is located. So the main CPU consuming oparations are file reading and writing. In case of HD I suppose you are waiting just when the plugin reads all the big piece of data .
The closer keeping fragment to the begining the faster the trimming process has to be.
Nevertheless we are trying to optimise the editing algorithms so in further versions there should be options for faster editing.