It sounds to me that what the OP is doing is not jumping an entire queue, but when they are second in the queue, moving past a slow-moving person who is first. That seems perfectly reasonable. As someone who, when traveling for business, takes a long time to prepare for x-ray (I carry a lot of electronic equipment), I normally encourage the person behind me to go first, while I set up my trays. If I am too pre-occupied to do that, I have no problem with the person behind me asking politely and going ahead. Why should the entire line behind me wait because I have to split my carry-on into 4 or 5 different trays?