gstreamer video slows down after awhile

Question

I use appsrc to push jpg images to create a live video. The video displays correctly for the first 5 - 10 seconds, but gradually lags behind and drops frames over time. Below is the code:

#!/usr/bin/env python3
import os
import sys
import gi
import numpy as np

gi.require_version('Gst', '1.0')
from gi.repository import Gst, GObject
import time
import random

class TestOverlay(object):
        
    def exit(self):
        self.pipeline.set_state(Gst.State.NULL)

    def __init__(self):
        # configure appsrc.
        self.number_frames = 0
        self.fps = 8
        secperframe = 1 / self.fps
        self.duration = secperframe * Gst.SECOND  # duration of a frame in Gst seconds (nanosec?)
        self.launch_string = 'appsrc name=source is-live=true format=GST_FORMAT_TIME ' \
                            '! jpegdec ' \
                            '! videoconvert ! videoscale ! video/x-raw,width=700,height=500' \
                            '! autovideosink'

        # setup and start a pipeline.
        self.pipeline = Gst.parse_launch(self.launch_string)
        appsrc = self.pipeline.get_child_by_name('source')
        self.pipeline.set_state(Gst.State.PLAYING)

        # start loop to push data through pipeline.
        cnt = 0
        while True:
            try:
                tstart = time.perf_counter()
                cnt += 1
                # this function returns a block of jpeg data.  
                # Basically, it copies data from a mmap (memory mapped) block.
                # And another app refreshes the block as a much higher rate than self.fps
                data = self.getbuf() 
                if data is None:
                    print(f'+++ counter: {cnt}, warning: no data')
                    time.sleep(secperframe)
                    continue
                else:
                    print(f'+++ counter: {cnt}, data len: {len(data)}')

                buf = Gst.Buffer.new_allocate(None, len(data), None)
                buf.fill(0, data)
                buf.duration = self.duration
                timestamp = self.number_frames * self.duration
                buf.pts = buf.dts = int(timestamp)
                buf.offset = timestamp
                self.number_frames += 1
                retval = appsrc.emit('push-buffer', buf)
                if retval != Gst.FlowReturn.OK:
                    print(f'+++ possible error: {retval}')

                # wait to make sure there are no more frames than self.fps.
                tduration = time.perf_counter() - tstart
                if tduration < secperframe:
                    remain = secperframe - tduration
                    time.sleep(remain)
                    #print(f'+++sleep: {remain}, {tduration}')
                else:
                    print(f'+++ getbuf falls behind {tduration}')
                    
            except Exception as e:
                print(f'+++ Exception: {e}')
                break

        self.exit()

Gst.init(None)

player = TestOverlay()

try:
    loop = GObject.MainLoop()
    loop.run()
except KeyboardInterrupt:
    print("Exit loop due to Ctrl+c.")
    player.exit()
    
sys.exit(0)         

I suspect the frame rate may be to slow that frames are piling up in the buffer, but not sure how to set the frame rate in the lauch_string. "caps" does not seem to work:

caps=video/x-raw,format=JPEG,framerate=8/1

because JPEG is not one of the supported format. Also try to play with 'block', 'leaky-type', etc. without any lucks. Any pointers are greatly appreciated.