My two pence worth.

If you jump up from a surface, you are jumping up relative to that surface, and will land back (allowing for losses due to air resistance, etc.) with the same speed you kicked off with, but in the opposite direction – i.e. The harder you kick off, the harder you land. You now have to add this onto the downward speed of the lift at the point of time you lift off from the lift (that is you frame of reference).

Thus, if at the point that your feet leave the floor of the lift, you were travelling upward at 15mph (just a random number), then (ignoring losses due to air resistance) you will land back on the floor with a downward velocity of 15mph.

If the lift was falling at a rate of 200 mph at the time you launched, then you will lift off at a nett speed of 185mph downwards, and land at a speed of 215 mph. If the lift is continuing to move downward at 200 mph, then your collision speed will be 15 mph downward; but if the lift has stopped because it has hit the ground, then your collision speed will be the full 215 mph (i.e. You may have delayed the impact, but when it happens, it will be worse).

No, I am not a plummeting lift expert, so the above is guesswork, but it is what I would expect from my naïve knowledge of such things.