ST STM32H743XI (ARM M7 based) MCU - ADC Configuration
In Reference manual section 24.3.13: Channel-wise programmable sampling time:


  Constraints on the sampling time for fast and slow channels: For each channel, SMP[2:0] bits must be programmed to respect a minimum sampling time as specified in the ADC characteristics section of the datasheets.


So difference between fast and slow channels is only in minimum sample time.

To have same conversion time check section 24.3.17: Timing in same datasheet. Easiest way would be to have all parameters equal (sample time, data resolution). 

If you actualy want to have same sampling frequncy on all channels then you only need to have conversion time smaller (and not equal) than period of ADC.