Why my convertion time is not correct when the sampling time is less than 84 Sampling time ?

Hello,

I'm using STM32f411RE with ADC and I would like to know why when the sampling time is less than 84, I don't find by calculation the same expected conversion frequency/time?

My calculation is Tconv = (3+12)/210000Hz = 714 ns so a frequency of 1.4 MHz. But in the measurement, I find a frequency of 260 kHz:

Here is my program:

/* Infinite loop */
 /* USER CODE BEGIN WHILE */
 while (1)
 {
 /* USER CODE END WHILE */
 
 /* USER CODE BEGIN 3 */
	HAL_ADC_Start(&hadc1);
 
	HAL_ADC_PollForConversion(&hadc1, 1);
	HAL_GPIO_TogglePin(GPIOA, LD2_Pin);
	res = HAL_ADC_GetValue(&hadc1);
	res = res * 3.3/4095;
 }
 /* USER CODE END 3 */

I use a led with the toggle function after each conversion.

Thank you in advance for your help.