it is correct,
you probably changed your source from w7sp0 to w7sp1,
or you change your w7sp1 source that belongs to higher level of distribution (enterprise ;) )
in all way,
this slow mount-unmount trouble was created by ms...

in any way,
there is solution

Use
\Utils\"Extracted_Wim_Folders"
hence your builds with whatever source will not have any delays
