Just because the processor runs at 100mhz doesn't mean it process 100 million instructions per second. It can take MANY clock cycles for specific instructions to execute well bellow the read rate of the flash memory. Depending on the processor while it may need a 100mhz clock it could only work at 25MIPS well within the range for simple flash memory. If you're using a chip that is that high speed that has to fetch instructions quickly it's simple, the flash is designed to be access that fast. There is no hard limit to flash memory access.