Test code for showing that it's not possible to keep 30 FPS over time

print log

tcambin serial="20810358" name=source ! video/x-raw,format=GRAY8,width=1920,height=1080,framerate=30/1 ! videoconvert ! appsink name=sink
Press Esc to stop
No image reveived 
--- FPS = 14.021855258540066
--- FPS = 14.048198373782677
--- FPS = 14.053658882220258
--- FPS = 14.06174532870176
--- FPS = 14.06463707090887
--- FPS = 14.064004083831776
--- FPS = 14.063271080079687
--- FPS = 14.063111375092966
--- FPS = 14.06380848443846
--- FPS = 14.06594175484966
--- FPS = 14.066781612567786
--- FPS = 14.065088965411785
--- FPS = 14.065515996072874
No image reveived 
No image reveived 
No image reveived 
late frame: 272.7081775665283 ms
No image reveived 
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late frame: 205.8720588684082 ms
No image reveived 
No image reveived 
late frame: 205.57355880737305 ms
No image reveived 
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No image reveived 
late frame: 272.97377586364746 ms
No image reveived 
No image reveived 
late frame: 204.75530624389648 ms
No image reveived 
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late frame: 273.043155670166 ms
No image reveived 
No image reveived 
late frame: 206.34746551513672 ms
No image reveived 
No image reveived 
late frame: 205.75237274169922 ms
No image reveived 
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late frame: 273.3330726623535 ms
No image reveived 
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late frame: 205.6136131286621 ms
No image reveived 
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late frame: 273.3936309814453 ms
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late frame: 205.76763153076172 ms
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late frame: 205.2288055419922 ms
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late frame: 273.12517166137695 ms
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late frame: 205.83701133728027 ms
No image reveived 
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program.py

import cv2

import numpy
import gi
from collections import namedtuple
from time import time

gi.require_version("Gst", "1.0")
gi.require_version("Tcam", "0.1")

from gi.repository import Tcam, Gst, GLib, GObject

CameraProperty = namedtuple("CameraProperty",
                            "status value min max default step type flags category group")


class TIS:
    'The Imaging Source Camera'

    def __init__(self, serial, width, height, numerator, denumerator, color):
        Gst.init([])
        self.height = height
        self.width = width
        self.sample = None
        self.samplelocked = False
        self.newsample = False
        format = "BGRx"
        if (color == False):
            format = "GRAY8"

        p = 'tcambin serial="%s" name=source ! video/x-raw,format=%s,width=%d,height=%d,framerate=%d/%d' % (
            serial, format, width, height, numerator, denumerator,)
        p += ' ! videoconvert ! appsink name=sink'

        print(p)
        try:
            self.pipeline = Gst.parse_launch(p)
        except GLib.Error as error:
            print("Error creating pipeline: {0}".format(err))
            raise

        self.pipeline.set_state(Gst.State.READY)
        self.pipeline.get_state(Gst.CLOCK_TIME_NONE)
        # Query a pointer to our source, so we can set properties.
        self.source = self.pipeline.get_by_name("source")

        # Query a pointer to the appsink, so we can assign the callback function.
        self.appsink = self.pipeline.get_by_name("sink")
        self.appsink.set_property("max-buffers", 5)
        self.appsink.set_property("drop", 1)
        self.appsink.set_property("emit-signals", 1)
        self.appsink.connect('new-sample', self.on_new_buffer)

    def on_new_buffer(self, appsink):
        self.newsample = True
        if (self.samplelocked == False):
            try:
                self.sample = appsink.get_property('last-sample')
            except GLib.Error as error:
                print("Error on_new_buffer pipeline: {0}".format(err))
                raise
        return False

    def Start_pipeline(self):
        try:
            self.pipeline.set_state(Gst.State.PLAYING)
            self.pipeline.get_state(Gst.CLOCK_TIME_NONE)


        except GLib.Error as error:
            print("Error starting pipeline: {0}".format(err))
            raise

    def Get_image(self):
        # Sample code from https://gist.github.com/cbenhagen/76b24573fa63e7492fb6#file-gst-appsink-opencv-py-L34
        if (self.sample != None and self.newsample == True):
            self.samplelocked = True
            buf = self.sample.get_buffer()
            caps = self.sample.get_caps()
            bpp = 4;
            if (caps.get_structure(0).get_value('format') == "BGRx"):
                bpp = 4;

            if (caps.get_structure(0).get_value('format') == "GRAY8"):
                bpp = 1;

            self.img_mat = numpy.ndarray(
                (caps.get_structure(0).get_value('height'),
                 caps.get_structure(0).get_value('width'),
                 bpp),
                buffer=buf.extract_dup(0, buf.get_size()),
                dtype=numpy.uint8)
            self.newsample = False
            self.samplelocked = False
            return self.img_mat
        return None

    def Stop_pipeline(self):
        self.pipeline.set_state(Gst.State.PAUSED)
        self.pipeline.set_state(Gst.State.READY)
        self.pipeline.set_state(Gst.State.NULL)

    def List_Properties(self):
        for name in self.source.get_tcam_property_names():
            print(name)

    def Get_Property(self, PropertyName):
        try:
            return CameraProperty(*self.source.get_tcam_property(PropertyName))
        except GLib.Error as error:
            print("Error get Property {0}: {1}", PropertyName, format(error))
            raise

    def Set_Property(self, PropertyName, value):
        try:
            self.source.set_tcam_property(PropertyName, GObject.Value(type(value), value))
        except GLib.Error as error:
            print("Error set Property {0}: {1}", PropertyName, format(error))
            raise


Camera = TIS("20810358", 1920, 1080, 30, 1, False)

cv2.namedWindow('Camera', 1)

# Start the pipeline
Camera.Start_pipeline()

cv2.waitKey(1000)

error = 0
print('Press Esc to stop')
lastkey = 0
KEYS = {'esc': 27, '1': 49, '2': 50, '3': 51}
try:
    last_frame_time = None
    frame_threshold_time = 0.2  # seconds
    inter_frame_time_average = 0
    frames = 0
    inter_frame_time_average_print_interval = 100  # measured in frames
    while (lastkey != KEYS['esc']):
        # Grab an image
        image = Camera.Get_image()

        if image is not None:
            error = 0
            # cv2.imwrite('img.jpg' , image)
            # show the image
            cv2.imshow('Camera', image)

            # Code for timing frames
            new_time = time()
            if last_frame_time is not None:
                # Look for lags
                inter_frame_time = new_time - last_frame_time
                if inter_frame_time > frame_threshold_time:
                    print(f"late frame: {(inter_frame_time) * 1000} ms")

                # Record inter-frame time
                frames += 1
                inter_frame_time_average = (inter_frame_time_average * (frames - 1)
                                            + inter_frame_time) / frames
                if frames % inter_frame_time_average_print_interval == 0:
                    print(f'--- FPS = {1 / inter_frame_time_average}')
            last_frame_time = new_time

        else:
            print("No image reveived ")
            error = error + 1

        # wait. The wait period should be somewhat longer than 1000 / fps.
        lastkey = cv2.waitKey(67)

        if lastkey == KEYS['1']:
            print(f'auto=True')
            Camera.Set_Property("Exposure Auto", True)
            Camera.Set_Property("Gain Auto", True)
            Camera.Set_Property("Focus Auto", True)
        elif lastkey == KEYS['2']:
            print(f'auto=False')
            Camera.Set_Property("Exposure Auto", False)
            Camera.Set_Property("Gain Auto", False)
            Camera.Set_Property("Focus Auto", False)
        elif lastkey == KEYS['3']:
            print(f'focus: {Camera.Get_Property("Focus Auto")}')
            print(f'expos: {Camera.Get_Property("Exposure Auto")}')
            print(f' gain: {Camera.Get_Property("Gain Auto")}')


except KeyboardInterrupt:
    cv2.destroyWindow('Window')

# Stop the pipeline and clean ip
Camera.Stop_pipeline()
print('Program ended')

This is not how video capture should work. The "last-sample" property of a sink element gives you exactly that: The last sample seen by the element. There is no queue of "last-sample" buffers in the element so you will just miss images.

The "opencv.py" example shows how to use the appsink element correctly: https://github.com/TheImagingSource/tiscamera/blob/master/examples/python/opencv.py

I modified this example to print the achieved frame rate every 30 frames here:
https://gist.github.com/TIS-Arne/76452cb32b3284984282da596ce3602a