At least with rtsp:// and satip:// Channels spawns an ffmpeg instance, I was going to have a play with the various options there to see if it sees the same latency/delay.
UPDATE:
I think the delay is coming from ffmpeg - if I run a basic test using the channels ffmpeg binary, it's around 7-8 seconds with BBC One HD before I see the "frame" in the log, i.e it's playing.
Testing with the same link in VLC, it starts playing within around 3-4 seconds - which is more or less the same as what I see when using TVHeadend -> Channels.
There are a variety of warnings/errors spat out from ffmpeg, although this was just a basic test (I'm not yet using the exact arguments that channels launches ffmpeg with, so it's not a like for like).
I need to do some research to see what these mean. Perhaps TVHeadend/VLC is more tolerant, or simply ignores, the errors or perceived 'dirty' stream?
[h264 @ 0x69b2f40] co located POCs unavailable
[h264 @ 0x7025fc0] reference picture missing during reorder
[h264 @ 0x7025fc0] Missing reference picture, default is 65724
[h264 @ 0x69b2f40] reference picture missing during reorder
Last message repeated 1 times
[h264 @ 0x69b2f40] Missing reference picture, default is 65740
Last message repeated 1 times
[h264 @ 0x695ad40] Missing reference picture, default is 65743
[h264 @ 0x7025fc0] mmco: unref short failure
[h264 @ 0x69b2f40] Reference 2 >= 2
[h264 @ 0x69b2f40] error while decoding MB 76 18, bytestream 4749
[h264 @ 0x69b2f40] concealing 5973 DC, 5973 AC, 5973 MV errors in B frame
[h264 @ 0x695ad40] co located POCs unavailable
[h264 @ 0x695ad40] concealing 7909 DC, 7909 AC, 7909 MV errors in B frame
[h264 @ 0x7028cc0] co located POCs unavailable
[h264 @ 0x7028cc0] concealing 7112 DC, 7112 AC, 7112 MV errors in B frame
[h264 @ 0x6a15cc0] co located POCs unavailable
[ac3 @ 0x6977800] exponent -2 is out-of-range
[ac3 @ 0x6977800] error decoding the audio block
Error while decoding stream #0:0: Error number -16976906 occurred
[h264 @ 0x7025fc0] error while decoding MB 36 16, bytestream -16
[h264 @ 0x7025fc0] concealing 6253 DC, 6253 AC, 6253 MV errors in B frame
[h264 @ 0x69b2f40] Reference 3 >= 2
[h264 @ 0x69b2f40] error while decoding MB 118 6, bytestream 1549
[h264 @ 0x69b2f40] concealing 7371 DC, 7371 AC, 7371 MV errors in B frame
[h264 @ 0x695ad40] concealing 7676 DC, 7676 AC, 7676 MV errors in B frame
[h264 @ 0x7025fc0] reference picture missing during reorder
Last message repeated 1 times
[h264 @ 0x7025fc0] Missing reference picture, default is 66028
Last message repeated 1 times
[h264 @ 0x7025fc0] Reference 2 >= 2
[h264 @ 0x7025fc0] error while decoding MB 49 0, bytestream 21350
[h264 @ 0x7025fc0] Cannot use next picture in error concealment
[h264 @ 0x7025fc0] concealing 8160 DC, 8160 AC, 8160 MV errors in P frame