Measuring the speed of an object moving fast through air
You are proposing calculating velocity based on sampled acceleration.  Essentially taking the integral of the acceleration.  This is the first step in a process call dead reckoning. This is discussed in this arduino.stackexchange.com answer.  In that answer there is a link to a web page which implements dead reckoning using an Arduino. You will find this equation on that web page:

velocity(i) = velocity(i-1) + acceleration (i)