Can't record audio fast enough

[jack]

Hello,

I am using pulseaudio (version 12.2) to "join" two audio streams created 
with gstreamer (1.14.4) on a raspberry pi (Raspbian GNU/Linux 10 
(buster)).

Here the real things :

In a first shell, i start with :
$ pacmd load-module module-null-sink sink_name=MySink

Then I use gstreamer to get the sound from my audio input (in an other 
shell) :
$ gst-launch-1.0 pulsesrc 
device="alsa_input.usb-0d8c_C-Media_USB_Headphone_Set-00.analog-mono" ! 
audioconvert ! audioresample ! pulsesink device=MySink

and I send this sound on an icecast server (in an other shell) :
$ gst-launch-1.0 pulsesrc device=MySink.monitor ! 
"audio/x-raw,channels=2" ! audioconvert ! audioresample ! opusenc ! 
oggmux ! shout2send ip=... port=... mount=... password=...

Everything is fine, but after 10 minutes, I get in the 3rd shell 
terminal ($ gst-launch-1.0 pulsesrc device=MySink.monitor ! ...) a 
sequence of messages like :

WARNING: from element /GstPipeline:pipeline0/GstPulseSrc:pulsesrc0: 
Can't record audio fast enough
Additional debug info:
gstaudiobasesrc.c(849): gst_audio_base_src_create (): 
/GstPipeline:pipeline0/GstPulseSrc:pulsesrc0:
Dropped 10080 samples. This is most likely because downstream can't keep 
up and is consuming samples too slowly.
WARNING: from element /GstPipeline:pipeline0/GstPulseSrc:pulsesrc0: 
Can't record audio fast enough
Additional debug info:
gstaudiobasesrc.c(849): gst_audio_base_src_create (): 
/GstPipeline:pipeline0/GstPulseSrc:pulsesrc0:
Dropped 34560 samples. This is most likely because downstream can't keep 
up and is consuming samples too slowly.
WARNING: from element /GstPipeline:pipeline0/GstPulseSrc:pulsesrc0: 
Can't record audio fast enough
Additional debug info:
gstaudiobasesrc.c(849): gst_audio_base_src_create (): 
/GstPipeline:pipeline0/GstPulseSrc:pulsesrc0:
Dropped 48000 samples. This is most likely because downstream can't keep 
up and is consuming samples too slowly.
...

The sound become glitchy and stop after 2 or 3 minutes.

According to the message, the stream after "pulsesrc 
device=MySink.monitor" is too slow. But I don't understand why.

Do you know where is the problem in this setup and how to solve it ?
Thanks any suggestion !
++

Jack