At what point does a HD slow down?

WD Internal Drives Desktop & Mobile Drives
1

I have a 86 year old client  that wants a 2nd HD install in his PC as he only has 200gigs free on his primary HD.  He keeps it well defrag’ed and uses it mostly for FlightSim (9 & 10).  He believes installing any addition data will slow his machine down. 

Is there a point where a HD slows down with to much data?  (keeping it defrag’ed)