Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Silicon Labs/SiM3_NRND/SIM3L166_C/FLASHCTRL_0/TCONTROL#0x0
FLRTMD=SLOW
Flash Timing Control
Flash Read Timing Mode.
0 (SLOW): Configure the flash read controller for AHB clocks below 12 MHz.
1 (FAST): Configure the flash read controller for AHB clocks above 12 MHz.
https://github.com/cmsis-svd/cmsis-svd-data