why adc is slower at 72mhz compared to 56mhz ?

[mehmetcanbalci]

in reference there is such information ;

STM32F103xx performance line devices: 
1 μs at 56 MHz (1.17 μs at 72 MHz)

why does it take longer in 72mhz ?

[Tesla DeLorean]

Because the max clocking rate for the ADC is 14 MHz, and the dividers get closer to that from 56 MHz than 72 MHz