STM32L0 ADC Interrupt Low Sampling Rate

Question

My observed sampling rate is much lower than expected and I cannot understand why.

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I have the discovery STM32L053C6 Discovery board and I want to use ADCs sampling. I started from the example STM32L053C8-Discovery/Examples/ADC/ADC_RegularConversion_Interrupt/ from STM32CubeL0. In main, I have the following ADC initialisation:

AdcHandle.Instance = ADC1;

AdcHandle.Init.OversamplingMode      = DISABLE;

AdcHandle.Init.ClockPrescaler        = ADC_CLOCK_SYNC_PCLK_DIV1;
AdcHandle.Init.LowPowerAutoPowerOff  = DISABLE;
AdcHandle.Init.LowPowerFrequencyMode = DISABLE;
AdcHandle.Init.LowPowerAutoWait      = DISABLE;

AdcHandle.Init.Resolution            = ADC_RESOLUTION_12B;
AdcHandle.Init.SamplingTime          = ADC_SAMPLETIME_7CYCLES_5;
AdcHandle.Init.ScanConvMode          = ADC_SCAN_DIRECTION_FORWARD;
AdcHandle.Init.DataAlign             = ADC_DATAALIGN_RIGHT;
AdcHandle.Init.ContinuousConvMode    = ENABLE;
AdcHandle.Init.DiscontinuousConvMode = DISABLE;
AdcHandle.Init.ExternalTrigConvEdge  = ADC_EXTERNALTRIGCONVEDGE_NONE;
AdcHandle.Init.EOCSelection          = ADC_EOC_SINGLE_CONV;
AdcHandle.Init.DMAContinuousRequests = DISABLE;

/* Initialize ADC peripheral according to the passed parameters */
if (HAL_ADC_Init(&AdcHandle) != HAL_OK)
{
  Error_Handler();
}


/* ### - 2 - Start calibration ############################################ */
if (HAL_ADCEx_Calibration_Start(&AdcHandle, ADC_SINGLE_ENDED) != HAL_OK)
{
  Error_Handler();
}

/* ### - 3 - Channel configuration ######################################## */
/* Select Channel 0 to be converted */
sConfig.Channel = ADC_CHANNEL_0;
sConfig.Rank = 0;
if (HAL_ADC_ConfigChannel(&AdcHandle, &sConfig) != HAL_OK)
{
  Error_Handler();
}

/* ### - 4 - Start conversion in Interrupt mode ########################### */
if (HAL_ADC_Start_IT(&AdcHandle) != HAL_OK)
{
  Error_Handler();
}

And the call back as

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef* AdcHandle)
{
  /* Get the converted value of regular channel */
  uwADCxConvertedValue = HAL_ADC_GetValue(AdcHandle);
  HAL_GPIO_TogglePin(GPIOB,GPIO_PIN_4);
}

I have the system clocks defined as

static void SystemClock_Config(void)
{
  RCC_ClkInitTypeDef RCC_ClkInitStruct;
  RCC_OscInitTypeDef RCC_OscInitStruct;

  RCC_OscInitStruct.OscillatorType  = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState        = RCC_HSI_ON;
  RCC_OscInitStruct.PLL.PLLSource   = RCC_PLLSOURCE_HSI;
  RCC_OscInitStruct.PLL.PLLState    = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLMUL      = RCC_PLL_MUL4;
  RCC_OscInitStruct.PLL.PLLDIV      = RCC_PLL_DIV2;
   if(HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    /* Initialization Error */
    Error_Handler();
  }

  RCC_ClkInitStruct.ClockType = (RCC_CLOCKTYPE_SYSCLK | RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2);
  RCC_ClkInitStruct.SYSCLKSource =  RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV2;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV2;
  if(HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK)
  {
    /* Initialization Error */
    Error_Handler();
  }
}

And the MSP init as

void HAL_ADC_MspInit(ADC_HandleTypeDef *hadc)
{
  GPIO_InitTypeDef                 GPIO_InitStruct;

  /*##-1- Enable peripherals and GPIO Clocks #################################*/
  /* Enable GPIO clock ****************************************/
  __HAL_RCC_GPIOA_CLK_ENABLE();
  /* ADC1 Periph clock enable */
  __HAL_RCC_ADC1_CLK_ENABLE();

  /*##- 2- Configure peripheral GPIO #########################################*/
  /* ADC3 Channel8 GPIO pin configuration */
  GPIO_InitStruct.Pin = GPIO_PIN_0;
  GPIO_InitStruct.Mode = GPIO_MODE_ANALOG;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*##-3- Configure the NVIC #################################################*/
  /* NVIC configuration for ADC EOC interrupt */
  HAL_NVIC_SetPriority(ADC1_COMP_IRQn, 0, 0);
  HAL_NVIC_EnableIRQ(ADC1_COMP_IRQn);
}

What that configuration, I would expect the conversion time to be at about 1.3µs. However, the PB4 toggles every 11µs. Which means that I get a sampling rate slightly above 80kS/s.

The manual mentions sampling rates above 1MS/s. I am sure that I am missing something, but cannot see what that would be. Or should I use DMA modes to get to higher sampling rates?