hs.window.filter.default:getWindows() is slow

I'd like to use hs.window.filter.default:getWindows() as a pretty key part of my workflow to flip back to the previously-focused window, but it's very inconsistent in terms of performance. Delays can be 2s frequently, up to 5s. Anyone else seeing performance problems with this particular function? Anything I can do to make it snappy enough to use?