Ffmpeg too slow when simultaneously splits and reencode videos

[kromak]

Hello, I frequently only need to reencode part of video files, so I try to use the ffmpeg command to split while also instruct ffmpeg to reencode it. Something like this:

ffmpeg -i input.mkv -c:a libmp3lame -b:a 48k -ac 1 -vf scale=720:-2 -c:v mpeg4 -vtag xvid -b:v 969k -ss 2:19:00 -t 3000 input.avi

The problem is that when I try to do so, it seems ffmpeg becomes stuck in a line like this

frame=    0 fps=0.0 q=0.0 size=      10kB time=00:00:00.00 bitrate=N/A speed=   0x

Due to the enormous time it take when the starting point is far away from the beginning, the impression I have is that ffmpeg is re-encoding since the start of the video file, which is perhaps quite unnecessary?

I don't know a way to avoid it, except by first to split the video, and then re-encode the second half. Which, unfortunately, is usually not acceptable, since I frequently do not have the spare space to store the intermediate split file so.