ffmpeg alternative to setpts for making a survey gif I have a 30fps video that is 234 minutes long and want to shrink it down to 2 minutes.  1-(234-2)/234) gives me 0.0085 which I use 

ffmpeg -i source.mp4 -vf setpts=PTS*0.0085,palettegen palette.png 

followed by 

ffmpeg -i source.mp4 -i palette.png --filter_complex setpts=PTS*0.0085,paletteuse survey.gif

Actually making the gif is excruciatingly slow compared to extracting the desired frames into a massive directory of png's which I can then combine with convert into a gif.  Aside from my main question, it doesn't seem like ImageMagick's convert supports user supplied palette files.

I understand that PTS is just adjusting how long a single frame is shown in a video but setpts has felt like the only reliably precise way of cutting the source.mp4 down.  

I looked at framestep and it is much better for speed but I couldn't figure out an ideal value for framestep that wouldn't make the resulting gif step from one frame to the next like what I would get from using convert and a delay of 0.5 between frames.

Aside from above, these survey are meant to quickly scan through hours of video for various points of concern in a gif format able to be uploaded privately to imgur or giphy so others on my team could review them remotely and without ftp/dropbox/etc.
Assuming you want a 30 fps 2.5m GIF, use

ffmpeg -i source.mp4 -vf "select='not(mod(n,trunc(234/2.5)))',palettegen" palette.png


and

ffmpeg -i source.mp4 -i palette.png -filter_complex "[0]select='not(mod(n,trunc(234/2.5)))',setpts=N/30/TB[v];[v][1]paletteuse" survey.gif