QSPI wont bring you the performance of the internal flash, but to provide even a rough analysis I would need the timing settings of the QSPI and somewhat more defined timing parameters than "for a very short time".
Move the vector table and the EXTI handler to the ITCM RAM. The vector table can be copied as it is, and SCB->VTOR adjusted when it is in place. Moving the function requires a bit of work.
Check that ITCM is defined in the linker script (*.ld)
MEMORY
{
DTCMRAM (xrw) : ORIGIN = 0x20000000, LENGTH = 128K
ITCMRAM (xrw) : ORIGIN = 0x00000000, LENGTH = 64K
It should be already there, copy the ITCMRAM line there if it is not.
Then define the section that goes into ITCM
_siccmram = LOADADDR(.ccmram);
.ccmram :
{
. = ALIGN(4);
_sccmram = .; /* create a global symbol at ccmram start */
*(.ccmram)
*(.ccmram*)
. = ALIGN(4);
_eccmram = .; /* create a global symbol at ccmram end */
} >ITCMRAM AT> FLASH
insert it after the .bss block in the linker script.
Then in the source code, mark the EXTI handler and whatever functions it calls so that they go to CCM.
__attribute__((section(".ccmram")))
void EXTIx_Handler(void) {
/* do stuff */
}
and make sure that you copy them into CCM before enabling the interrupt.
extern char _sccmram, _eccmram, _siccmram;
memcpy(&_sccmram, &_siccmram, &_eccmram - &_sccmram);
__DSB();__ISB(); // before possibly jumping to technically self modifying code
