There is something called After market order that u can put
Or you could just press Market order the moment u see price change.
fastest time i ever received was
-- 13Feb2017,09:15:11 -- NFO MARKET ORDER : Status: put order req received SELL NIFTY17FEB8800CE Total QTY: Disc. QTY:0 Open QTY:9600 Filled QTY:0 at Market Price 0.00 DAY
-- 13Feb2017,09:15:13 -- NFO MARKET ORDER fill Status : open SELL NIFTY17FEB8800CE Total QTY:9600 Disc. QTY:0 Open QTY:8775 Filled QTY:825 at Market Price
so fastest order possible for we retailers is 13 sec after market opens