perhaps its all "in the mind" as the saying goes? sex is both a physical and mental thing.
with your SO, its different because she is doing it for pleasure and hence, the mental
factor affects the physical state of being. hence, she feels tired because her mind is
exhausted from the exercise.
for a WG, they may just look at it as a job. hence, there is very little mental workout
involved for them. yeah i'm sure they will get physically tired as well (especially the newbies
to the trade). i don't suppose they take health supplements or health tonics
to maintain/increase their stamina? some probably do i guess.
WGs are human as well and working long hours do take a toll on them. A late night booking
may mean she would be more worn out and therefore less enthusiatic. Again that depends
on her mood and her working attitude, whether you are a regular etc.
i'm not a 'super-experienced' punter and this is just my take on the question posed.