Even with Extremis, that's still nowhere near lightspeed. Batman tried to prep specifically for Barry, and even then acknowledged it had to assume he wasn't moving at optimal speeds:
Extremis was a really good read. Regarding the superspeed aspect of that, he upgraded himself/the suit to cope with the heightened speed and reflexes of that KKK-wannabe that had messed him up in their first fight. The upgrades were successful, and Tony won.
It’s just that the speeds that enhanced human was using against Tony were slow compared to someone like the Flash.
The thing about the Flash is it isn't even just speed, his powers are much more exotic than that, a lot of physics really doesn't apply to him except when he wants it to. Flash Fact, someone moving at the speed of sound is standing still to someone in the speed force. In fact, someone moving at the speed of light is standing still to someone in the speed force. The flash is almost impossible to beat in forum debates. I got banned a couple of times in the last decade for threads where the flash moving at top speed with items like the ebony blade is unleashed on Marvel Earth, how long does it take for him to kill every human on the planet.