Silicon drift detectors (SDDs) are a well-established technology that has revolutionized spectroscopy in fields as diverse as geology and dentistry. At a first glance it would seem that detectors with such a slow response would not be suitable for the new ultra-fast x-ray free-electron lasers (FEL) coming online. However, for a subset of experiments at FELs, SDDs can make substantial contributions. Many measurements involve only several distinct photon energies known a priori, allowing pile-up deconvolution and accurate spectroscopic photon counting. Often the unknown spectrum is interesting, carrying science data, or the background measurement is useful to identify unexpected signals. We investigated the performance of SDDs at x-ray FELs, in particular the ability to deconvolve the spectrum that results from various combinations of a few wavelengths and the possibility of separately recording photons that are absorbed at different radii (thus having varying drift times). The analytic approach presented here permits isolating individual photon energies and interaction radii from pile-up events of 0 to 5 photons sampled from 6 monochromatic lines in a single SDD with accurate pile-up deconvolution, timing extraction and clipping correction. The usefulness of SDDs will continue into the x-ray FEL era of science. Their successors, the ePixS hybrid pixel detectors, already offer hundreds of pixels with similar performance in a compact, robust and affordable package.