Light coming from the Sun takes just over 8 minutes to reach us. Some very energetic particles from solar flares travel at a large fraction of the speed of light. They reach us very quickly (anywhere from 15 minutes to a few hours after the flare). These high-energy particles present the greatest danger to astronauts.
The International Space Station is being assembled in orbit. This work will require several thousand hours of space walks by astronauts. If a solar flare or CME occurs during one of those space walks, it could be a serious health risk.