A car window that moves slowly can be quite annoying, and if not fixed may stop working altogether. A power window may move slowly for a number or reasons, however most fixes for a slow power window are quick and inexpensive.
A low battery may make the power windows move slowly. A low battery will typically affect all of the windows and may be noted in the operation of other electrical devices, as well. A car with a low battery should be tested at a repair or parts store to determine the source of the problem.