Especially if you're already going as hard and deep as you can.

... no pun intended (ok, sorta) and needs corroboration from the ladies:  I'm wondering if this request has something to do with the potential positive effect (for the woman) of balls-on-clit, and "harder" will mean more stimulation there?

Or as you said, maybe she just wants to try to make us cum sooner.  

I have not met many who use this phrase but yes I do believe it is to get the person to cum faster. It fits well with other little things I notice that support the same conclusion

then you're probably boring me.
If I'm telling you to go harder, then I definitely want to enjoy it too.

One gal was sucking my cock and I slapped her ass.  She looked at me and said, "Harder!"
I was fucking another in standing dog next to a mirror and she yelled, "Pound me!"
There are more, but you get the idea.

take her and be more dominant, manly and in control. No woman needs to be kept on some fragile pedestal.

Honestly, there is a time for everything. Sometimes romantic, slow and gentle sex; other times hard and fast fucking; other times playful sex and laughter.  

You want to be fucked well, and so does she. That doesn't mean the same thing every day or every time. Being fucked well is both of you getting what you need when you need it.

Reading one another and communicating is key. Thank god for women who express what they need.

Harder and faster doesn't necessarily mean you have to lose control faster. There are a lot of ways to last longer if you choose. Find positions where your muscles are more relaxed; relaxed muscles will help you last longer. Try changing positions more frequently when you near the edge. A few seconds during the changeup will let you last longer.  

So no, I don't think she is trying to make you cum faster. I think she wants you both to be fucked well. That's a great thing

her harder and deeper. Sometimes they cannot take it harder. If you pound on them to hard they may not see you again.

Then telling me to go "harder" or "faster" is the wrong thing to do.   It will make ME cum harder and faster, not her.