Suddenly too slow at work?

Hello everybody ...

At this point, I would like to raise another issue, as an addition to a problem, which I recently mentioned here in advance.
I had recently reported from my superiors, with whom I quarrel for over 2 months, whereby we exchange since then only about the essentials and just just professional. On the whole, that also works quite well, although I also feel that the situation has relaxed a little bit.
Now, though, behind my back she complains to my boss's wife, with whom I have a great deal of sympathy, that I'm supposed to make my job too slow. The background is the fact that on Saturdays we have an increased customer frequency at times and we often have to serve the customers all alone in the department. However, it sometimes happens that one or the other customer complains about the fact that only one person alone serves the customers. Against this background, my boss, so the manager my supervisor addressed that it can not be that Saturday lunchtime, where it is sometimes fuller, almost always only one does his job. He has made her understand that she would like to try for another work plan to counteract such problems in the future.
So, first of all, my supervisor did her duty alone at lunch today, until I got in the afternoon. You will not believe it, but "by chance" she did not have much clientele at the time, so it was very quiet.
After about 1 hour, she made her break and you will not believe it, within 2 minutes the department was full, as if a bus had pulled up !! OK, she interrupted her break for a while and helped me to serve the customers.
Well, when she went back into the break room and at the desk of our boss's wife, she said to her at the moment that I'm probably too slow at work. Firstly, I am NOT too slow and secondly I can not help it if in the very same moment suddenly so many customers come, where I'm just alone in the department again !! Incidentally, there are even witnesses to these "strange" circumstances !! Apart from that, I will soon be in this company for 10 years, working with my so-called superiors over 5 years together.
And somehow I'm too slow at work ... and strangely, such statements always come just when there are major problems or I am in dispute with my colleague? And above all ... why does not she remind me herself that she is not satisfied with my performance, so that I have any chance to think about it and work on it, if that is necessary at all? In addition ... if she has serious and well-founded concerns ... why does not she then go to the next superior manager or directly to the boss, at this point my work performance in question and if necessary. To be checked? But no, I hear it from a completely different source and nothing happens!
I have to say that I understand my boss, his wife and all other superiors very well ... only with her I have problems again and again. In addition, I have been informed from a higher, secured source that the boardroom, the negative characteristics of my colleague or superiors, but I do not want to list all here again, are quite well known. My boss is happy that he has them, but I would like to keep the reasons for this better for myself ... ;-)
But ... what is going on in this person's mind and, above all, what is she aiming at? I have to say that she is very moody and often lets others feel when she is not feeling well or when she has ALSO PRIVATE problems. As long as everything goes as she imagines and wants, the world is in order and you can get along quite well. But woe there are problems ... technically and also private, but then Holland is in need !!!
What do you think about this and how would you handle it?

Thank you for your suggestions