One often overlooked component in power steering systems, is the filter.  I
know this sounds strange but most systems have a filter in the reservoir,
normally retained against the tank wall at the return line port by a coil
spring.  I have fixed many sluggish power steering systems by merely
changing this filter along with the  fluid.   I do  not know that VW  has a
filter in their system however.  But it is likely there is one there
somewhere.  If it gets partially clogged it hampers efficiency, coming on
gradually as you have described.  I hope this problem you have turns out to
be this simple. If there is no filter then a check of the flow  on the
return at the  reservoir should eliminate  this possible cause.  The steel
line can be crimped or the rubber hose can be collapsed as brake hoses
often fail this way.
Good luck with it.
  Jeff/Florida