Loop the first frame of a video and then play a selected portion of the video
A shorter method is

ffmpeg -i video.mp4 -vf 'select=eq(n,0)+between(n,310,930),setpts=if(eq(N,0),0,(N+260)/FRAME_RATE/TB)' -map [output] -vsync cfr -c:v libx264 -crf 18 -y output.mp4