To achieve your goal of reducing the number of frames in an image sequence while maintaining the same duration, you can manually remove the frames you don't need from your sequence. However, this will result in "missing images" in Spine if the sequence expects those frames to be present. A workaround is to adjust the sequence settings to skip frames by setting the FPS of the sequence key to half of the timeline FPS. For example, if your timeline FPS is 30, set the sequence key FPS to 15. This way, the sequence will play every other frame, effectively reducing the number of frames used while maintaining the same playback duration. Unfortunately, Spine does not currently have a built-in feature to automatically skip frames during export without causing missing images.

@plunksville Note that you have to be in Animate mode with the attachment selected to see the image sequence FPS, as loop, FPS etc can be set differently for each animation, and can even be keyed to change the speed in the middle of an animation.

To correct what Spinebot is saying, if you set the FPS to 15 instead of 30, your sequence will still go in order and not skip numbers. I suppose your sequence was output from somewhere like AE, so you could export at a lower FPS from the source fo your sequence and then match the FPS in Spine. Or if you have time to spend, you could also rename the files manually, but probably acting from the source would be more practical.