How to calculate the FLASH writing time of MKV3x serious microcontroller? The formula for that. Why is it not working in high speed mode?
Flash write times can be found in the data sheet. However they vary between chips and change with age and so can not be considered to be a fixed value - instead you need to consider the maximum value that is given.
In HSRUN mode flash operations are not possible - they are blocked. I don't know the physical reasons for this.
[uTasker project developer for Kinetis and i.MX RT]
Some Flash technologies are slow compared to the reset of a given architecture, so various work arounds are implemented, copy to RAM and run from there, wait states, don't allow etc.
https://www.renesas.com/us/en/ Rx parts have a faster Flash technology than most other implementations, so don't run into until a lot higher frequencies.