...

def _get_video_thumb(path: str, tiny: bool = False):
    ## <-- New requests stop being processed here -->
    vid = cv2.VideoCapture(path)
    vid.set(cv2.CAP_PROP_POS_FRAMES, (int(vid.get(cv2.CAP_PROP_FRAME_COUNT)) // 3) - 1)
    ret, frame = vid.read()
    ## <-- Requests resume here -->
    if not ret:
        raise RuntimeError('failed to read video frame')
    img = Image.frombytes('RGB', (frame.shape[1], frame.shape[0]), cv2.cvtColor(frame, cv2.COLOR_BGR2RGB))
    with BytesIO() as bio:
        img.save(bio, format='JPEG')
        bio.seek(0)
        return bio.read()

...

@app.route('/_/thumbnailer')
async def thumbnailer():
    full_path, check = await verify_path(request.args.get('path', None))
    if not check:
        await abort(500)
    i = await run_sync(func)(path=str(full_path), tiny=request.args.get('scale', False, type=lambda v: v.lower() == 'true'))
    return await make_response(i, 200, {'Content-Type': 'image/jpeg'})

...