I would say there is a difference between discomfort, and pain. Not to mention if there is pain, severe pain is just around the corner if you get more vigorous.
If she is showing discomfort, the best thing to do is add lube, (there can not be too much lube with anal, tearing/ripping/irritation from friction is a high risk.) Hold still for a moment to give her body a chance to relax enough to ease the discomfort, then keep going when ready, vigorously won't hold the same risk of damage and healing time needed after, if it is properly taken care of during.
If you take a moment to care for her needs, it can get so much more pleasurable than uncomfortable. If she looks uncomfortable, it isn't feeling good, she just wants you done asap.
Going more vigorously when she is already showing discomfort can cause damage and it can put her out of commission for a while. Healing there can last several days, where every bathroom visit could lead to tears, even more while excessive cleaning, a tear isn't on the external surface, but internally, harder to disinfect and treat. Not to say anything about the damage that can be caused to the intestines ability to function correctly.
Don't mistake her desire not to ruin your paid expierience, as her enjoying. Her showing discomfort is her true reaction, and if she is enjoying, it souldn't come off as discomfort, unless that is her O face.
Your description of how you enjoy anal is one of the top reasons I refuse to offer it to clients. It needs proper care.
