[FFmpeg-user] A question about adding delay in a live streaming

[xyzero]

Hi all:
    I want to add a 5s delay in a live streaming such as rtmp, and I use ffmpeg command as follows
    "ffmpeg -re -i rtmp://live.hkstv.hk.lxdns.com/live/hks -muxdelay 5 -c copy -f flv rtmp://localhost/rtmp/test"
The result is the delay has no effect.
If I change the command param "-c copy" to "-c:v h264 -c:a aac", then it works correctly in 5s delay.
Delay in a live streaming must need transcoding?
If transcoding is unavoidable, how to use min CPU load to add the delay?
Thanks!