Pygame is actually a lot faster than I thought it would be.

pygame-fps.py

#!/usr/bin/python3

"""
Python 3.6 or above.
Cygwin: startxwin ./pygame-fps.py
"""

import numpy
import pygame
import time

START_SIZE = (1155, 866)  # (width, height) for roughly one 4:3 megapixel
FPS_DATA_LENGTH = 50  # How many frames to average when measuring FPS
COLOR_MAX = 256 ** 3  # Full 24-bit RGB color spectrum

surface = pygame.display.set_mode(START_SIZE, pygame.RESIZABLE)
surfsize = surface.get_size()

fps_data = numpy.full(FPS_DATA_LENGTH, numpy.inf)
fps_index = 0
done = False

while not done:
    fps_start_time = time.time()

    pygame.display.set_caption(
        f'{FPS_DATA_LENGTH / sum(fps_data):.1f} fps @ {surfsize[0]}x{surfsize[1]}')
    pygame.surfarray.blit_array(surface, numpy.random.randint(0, COLOR_MAX, surfsize))
    pygame.display.update()

    for event in pygame.event.get():
        if event.type in (pygame.KEYUP, pygame.QUIT):
            done = True
        elif event.type == pygame.VIDEORESIZE:
            surface = pygame.display.set_mode(event.size, pygame.RESIZABLE)
            surfsize = surface.get_size()

    fps_data[fps_index] = time.time() - fps_start_time
    fps_index = (fps_index + 1) % FPS_DATA_LENGTH