OpenCV access live streaming from FFserver delay at start - opencv

Question

I've already posted this question on superuser.com (https://superuser.com/questions/1718608/ffmpeg-live-streaming-to-ffserver-delay-at-start), but since part of this question is OpenCV related, they suggested me to post it also here.
I'm trying to achieve a simple camera streaming using FFMpeg and FFserver. I have two slightly different systems acting as source, both Debian OS:
the first one runs ffmpeg 3.4.8, as indicated in figure 1
First system FFMPEG version
the second one runs ffmpeg 2.8.17, as indicated in figure 2
Second system FFMPEG version
The ffmpeg command used to send the stream to to ffserver is the following, identical for both systems:
ffmpeg -re -f v4l2 -s 640x360 -thread_queue_size 20 -probesize 32 -i /dev/video0 -threads 4 -fflags nobuffer -tune zerolatency http://myserverIP:myserverPort/liveFeed.ffm
In order to see the stream result I access the live stream from a third system using openCV pointing to the server URL:
VideoCapture videoCap = new VideoCapture("http://myserverIP:myserverPort/liveFeed.flv");
...
videoCap.read(imageInput);
and start grabbing the incoming frames from the stream.
The wierd thing happens here:
with the first system the video stream visualized through openCV is pretty much real time, with 1-2 seconds of delay from the original source.
with the second system the video stream is affected by a variable delay which is comparable with the elapsed time between the start time of the stream source and the start time of the stream acquisition with openCV (for example: if I start the source stream at 12:00:00 and wait 30 seconds before access the stream with openCV, I have a delay of about 30 seconds shown on the third system)
The ffserver configuration is the following
HTTPBindAddress 0.0.0.0
MaxHTTPConnections 2000
MaxClients 1000
MaxBandwidth 6000
CustomLog -
#NoDaemon
<Feed liveFeed.ffm>
File /tmp/SCT-0001_3.ffm
FileMaxSize 5M
</Feed>
<Stream liveFeed.flv>
Format flv
Feed liveFeed.ffm
VideoCodec libx264
VideoFrameRate 20
VideoBitRate 200
VideoSize 640x360
AVOptionVideo preset superfast
AVOptionVideo tune zerolatency
AVOptionVideo flags +global_header
NoAudio
</Stream>
##################################################################
# Special streams
##################################################################
<Stream stat.html>
Format status
# Only allow local people to get the status
ACL allow localhost
ACL allow 192.168.0.0 192.168.255.255
</Stream>
# Redirect index.html to the appropriate site
<Redirect index.html>
URL http://www.ffmpeg.org/
</Redirect>
Any help to spot the problem would be great! Thanks