gstreamer v4l2 sync parameter meaning

Hello Dear community,
I am using gstreamer to stream files from  disk to  (v4l2)  video device.
the gst-launch commands (below) seems to work for small files.

However, when i run the command with  4k files ,for example, i get the
error:
"There may be a timestamping problem, or this computer is too slow.".

when i add  sync = 0 to the end of the gst-launch commands (below), the
error disappers.

could anyone  please advise:
1. what cases the error ?
2. what is the meaning of the fix ?
    what am i loosing by setting   "v4l2sink sync=0" \   "xvimagesink
sync=0"  ?
   does that mean theat gstreamer does not guantee to the frame rate
specified in the command  (30/1) ?

   PS: documentation only tells "Sync on the clock"


Thank you !!!

More details:

command to inject to screen:
gst-launch-1.0 -v multifilesrc location=./%04d.png loop=1
caps="image/png,framerate=30/1" ! pngdec ! videoconvert  ! xvimagesink

command to inject to virtual video device:
 gst-launch-1.0 -v multifilesrc location=./%04d.png loop=1
caps="image/png,framerate=30/1" ! pngdec ! videoconvert  !
"video/x-raw,format=YUY2" ! v4l2sink device=/dev/video11


Detailed error:
There may be a timestamping problem, or this computer is too slow.
                WARNING: from element
/GstPipeline:pipeline0/GstV4l2Sink:v4l2sink0: A lot of buffers are being
dropped.
                Additional debug info:
                ../libs/gst/base/gstbasesink.c(3143):
gst_base_sink_is_too_late (): /GstPipeline:pipeline0/GstV4l2Sink:v4l2sink0: