High CPU load and lags when playback the video with EGLFS via X11 backend

Attempt to play a QML video lead to the high CPU load ~300% (where from the 4-cores, the 3 - cores has ~100% load). Also I see in console this output:

Warning: "A lot of buffers are being dropped."
Warning: "A lot of buffers are being dropped."
Warning: "A lot of buffers are being dropped."

and the AVI video has ~0.3-0.5 FPS.

The gst-inspect shows this video sinks:

root@apalis-t30:~/Downloads# gst-inspect | grep sink
ximagesink: ximagesink: Video sink
autodetect: autovideosink: Auto video sink
autodetect: autoaudiosink: Auto audio sink
playback: playsink: Player Sink
omx: nv_omx_audiosink: OpenMAX IL audiosink element
omx: nv_gstbin_videosink: OpenMAX IL videosink element
omx: nv_omx_videosink: OpenMAX IL videosink element
omx: nv_omx_hdmi_videosink: OpenMAX IL hdmi videosink element
omx: nv_omx_overlaysink: OpenMAX IL overlaysink element
omx: nv_gl_eglimagesink: OpenMAX IL videosink element
alsa: alsasink: Audio sink (ALSA)
tcp: tcpclientsink: TCP client sink
tcp: tcpserversink: TCP server sink
tcp: multifdsink: Multi filedescriptor sink
gio: giosink: GIO sink
gio: giostreamsink: GIO stream sink
nvxvimagesink: nvxvimagesink: Video sink
udp: udpsink: UDP packet sender
udp: multiudpsink: UDP packet sender
udp: dynudpsink: UDP packet sender
app: appsink: AppSink
xvimagesink: xvimagesink: Video sink
coreelements: fakesink: Fake Sink
coreelements: fdsink: Filedescriptor Sink
coreelements: filesink: File Sink
root@apalis-t30:~/Downloads#

[1] Then I try to play the video using the vendor-specific 'nvgstplayer' utility with different NV-based video sinks, I got the CPU load ~50%-100% (depends on video format), that it is normal (seems, the HW acceleration is enabled):

$nvgstplayer -i movie.avi --svs="nv_omx_hdmi_videosink" (~100%)
$nvgstplayer -i movie.mp4 --svs="nv_omx_hdmi_videosink" (~50%)

$nvgstplayer -i movie.avi --svs="nv_gl_eglimagesink" (~200%)
$nvgstplayer -i movie.mp4 --svs="nv_gl_eglimagesink" (~150%)

$nvgstplayer -i movie.avi --svs="nvxvimagesink" (~120%)
$nvgstplayer -i movie.mp4 --svs="nvxvimagesink" (~50)

that is important - I have no lags in video!!!

[2] Then I try to play a video, using the cpp-based QMediaPlayer class. For this case I set the QT_GSTREAMER_WINDOW_VIDEOSINK and QT_GSTREAMER_WIDGET_VIDEOSINK variables, which contains a video sink name:

export QT_GSTREAMER_WINDOW_VIDEOSINK=
export QT_GSTREAMER_WIDGET_VIDEOSINK=
$./cpp-video-player movie.mp4 (~300%, some lags)
$./cpp-video-player movie.avi(~300%, very big lags)

export QT_GSTREAMER_WINDOW_VIDEOSINK=nv_omx_hdmi_videosink
export QT_GSTREAMER_WIDGET_VIDEOSINK=nv_omx_hdmi_videosink
$./cpp-video-player movie.mp4 (~50%, no lags)
$./cpp-video-player movie.avi(~150%, no lags)

$export QT_GSTREAMER_WINDOW_VIDEOSINK=nv_gl_eglimagesink
$export QT_GSTREAMER_WIDGET_VIDEOSINK=nv_gl_eglimagesink

$export QT_GSTREAMER_WINDOW_VIDEOSINK=nvxvimagesink
$export QT_GSTREAMER_WIDGET_VIDEOSINK=nvxvimagesink

it does not starts at all, returns:

X Error of failed request: BadWindow (invalid Window parameter)
Major opcode of failed request: 3 (X_GetWindowAttributes)
Resource id in failed request: 0x3
Serial number of failed request: 7
Current serial number in output stream: 8

[3] Next, I had to use a simple QML player (the Video item), with the same set of QT_GSTREAMER_ variables.. But in this case it does nothing, I have very big CPU load and lags:

$./qml-video-player movie.mp4 (~300%, some lags)
$./qml-video-player movie.avi(~300%, very big lags)

So, seems this env variables are ignores for QML!!!

How it can be avoidet? Is there are any workaround?