I have put all clock configuration to its default state, so an internal oscillator is running at 8 mhz. I have a delay loop using inline assembly as follows:

// Delay a certain number of cycles using glorious inline assembly.
void delay(uint32_t time) {
    asm volatile(
        "mov r4, #3                \n" // Divide time by three since the loop is
        "udiv %[time], %[time], r4 \n" // 3x too slow.
        "loop:                     \n"
        "subs %[time], %[time], #1 \n" // 1 cycle
        "bne loop                  \n" // 1 cycle if not take, 2 if taken.
        : [time] "+l" (time)           // Put rw input variable time in r0..r7.
                                       // Make it rw and as output so we don't clobber
        :                              // Time is both input and output.
        : "r4", "cc"                   // We are clobbering r4 and condition code flags.
    );
}

and a GPIO routine as follows:

// Blink the led with a period of 1 second.
while (1) {
    // Set LED pin.
    GPIOA_BSRR = 1 << LED_PIN;
    delay(second_cycles / 2);

    // Reset LED pin
    GPIOA_BSRR = 1 << (LED_PIN + 16);
    delay(second_cycles / 2);
}

When running without wait states everything is fine and performing as expected. But when I enable change the wait states for flash from 0 to 1, my loop takes 833 milliseconds instead of 500 milliseconds, or a 66% loss in performance roughly.

When I use GDB to debug, I can see the FLASH_ACR register has the contents of 0b0011 0000 which signify that the prefetch buffer is both enabled and has a status of enabled, with 0 wait states for flash. When write to the register by oring it with 0b001 I get the expected result of 0b0011 0001 back after reading from it again.

Interestingly enough, enabling or disabling the prefetch buffer with a 1 wait state makes no difference in my loop, which doesn't seem to make sense.

I am using an NUCLEO-F303RE board, which uses an STM32F103RE.