Let's say it takes me about 1 sec to take a one regular step back.
Hong Kong, China is 8100 miles from New York. Let's say flash could run 17,000 miles a sec. That's over twice the distance, a half sec going and a half sec coming back. Going round-trip he'd be back before I complete my step back.
The speed of light is over 186,000 miles a SEC(670,616,629.384 miles per hour). That's WAAAAAAAAAAY faster than flash's measily 17,000 miles a sec.