This is all documented in the Reference Manual for your stm32.
There are different reference manuals for H750 and H730.
It's all pretty complicated (STCubeMX makes a good job of hiding the complexity, but I much prefer to know what/where the limits are. And it is possible that STCubeMX does not always follow all the correctly rules; programmers are only human).
The Kernel clock for a peripheral might be slower than the processor clock.
For example for H750 from table 59 on p352 of RM0433 many of the USARTs may be clocked up to 125 MHz at VOS0, but only 50 MHz at VOS2
And H730 in Table 56 on p335 of RM0468 says 137.5 MHz at VOS0 falling to 43 MHz at VOS3 (which sounds higher than H750)
I'll admit I have never tried to send data over UARTs that quickly; I guess you have to use DMA and have large, predictable blocks to transfer to make good use of such high data-rates.
Hope this helps,
Danish
