Thank you for your reply.
I tried tasks of your suggestion.
ryoyo@ubuntu:~$ gst-launch-1.0 rtspsrc location=rtsp://[appropriate config] ! rtph264depay ! h264parse ! decodebin ! fpsdisplaysink video-sink=fakesink text-overlay=false sync=false -v | grep current
NvMMLiteOpen : Block : BlockType = 261
NvMMLiteBlockCreate : Block : BlockType = 261
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 9, dropped: 0, current: 16.65, average: 16.65
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 29, dropped: 0, current: 13.87, average: 14.63
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 37, dropped: 0, current: 14.72, average: 14.65
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 57, dropped: 0, current: 14.55, average: 14.61
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 64, dropped: 0, current: 13.02, average: 14.42
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 85, dropped: 0, current: 14.57, average: 14.46
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 93, dropped: 0, current: 14.79, average: 14.48
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 113, dropped: 0, current: 13.90, average: 14.38
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 121, dropped: 0, current: 14.81, average: 14.40
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 141, dropped: 0, current: 14.50, average: 14.42
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 148, dropped: 0, current: 13.01, average: 14.34
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 169, dropped: 0, current: 14.56, average: 14.37
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 177, dropped: 0, current: 14.82, average: 14.39
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 197, dropped: 0, current: 13.88, average: 14.34
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 205, dropped: 0, current: 14.83, average: 14.36
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 225, dropped: 0, current: 14.49, average: 14.37
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 232, dropped: 0, current: 13.01, average: 14.32
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 253, dropped: 0, current: 14.55, average: 14.34
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 261, dropped: 0, current: 14.78, average: 14.35
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 281, dropped: 0, current: 13.90, average: 14.32
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 289, dropped: 0, current: 14.84, average: 14.34
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 309, dropped: 0, current: 14.49, average: 14.35
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 316, dropped: 0, current: 13.02, average: 14.31
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 337, dropped: 0, current: 14.56, average: 14.33
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 345, dropped: 0, current: 14.79, average: 14.34
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 365, dropped: 0, current: 13.90, average: 14.31
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 373, dropped: 0, current: 14.78, average: 14.32
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 393, dropped: 0, current: 14.50, average: 14.33
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 400, dropped: 0, current: 13.01, average: 14.31
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 421, dropped: 0, current: 14.56, average: 14.32
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 429, dropped: 0, current: 14.83, average: 14.33
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 449, dropped: 0, current: 13.89, average: 14.31
/GstPipeline:pipeline0/GstFPSDisplaySink:fpsdisplaysink0: last-message = rendered: 457, dropped: 0, current: 14.74, average: 14.32
As a whole, the performance was fine.
I think my difficulty is caused by using RTSP stream as a source.