F411 LL SPI-DMA callback delay too long

My code is based on LL SPI-DMA example.

The original code does not have chip select, so I use one GPIO pin as CS pin.

 /* Start SPI Tx/Rx */
 ADC_CS_Low();
 
 /* Enable DMA Channels to start Tx/Rx */
 LL_DMA_EnableStream(DMA2, LL_DMA_STREAM_2);
 LL_DMA_EnableStream(DMA2, LL_DMA_STREAM_3);
 
 /* Wait for previous Tx/Rx complete bf we send again
 */
 while(ub_SPI1_TxComplete != 1) { }
 ub_SPI1_TxComplete = 0;
 LL_DMA_DisableStream(DMA2, LL_DMA_STREAM_3);
 
 while(ub_SPI1_RxComplete != 1) { }
 ub_SPI1_RxComplete = 0;
 
 LL_DMA_DisableStream(DMA2, LL_DMA_STREAM_2);
 
 ADC_CS_High();

So the basic steps are:

1. CS Low
2. Enable Rx stream
3. Enable Tx stream
4. Wait for Tx/Rx complete signals, which are modified in Tx and Rx callback functions
5. Disable Tx/Rx stream
6. CS High

I used PicoScope to monitor clock, and CS signal.

I found from CS low to clock start the delay is well below 1 microsecond; while from clock end to CS high, it takes 2 microseconds. It's too long.

I wonder if there is any register value for me to check if Tx and Rx have finished; instead of replying on callback function.