Nike claims 4% increase in running economy, not speed. There's a huge difference.
I find holding a fast pace to be significantly easier in the 4% than any other shoes including my Turbo. I don't heel strike nor do any of the top athletes who race in them.
The carbon plate helps on pushoff. Honestly I'm not surprised many don't feel the difference because it really changes depending on pace and probably form.
Compared to the Turbo, I don't much of a difference at 8 min/mi, feel a noticeable difference at 7:15-7:30 pace (IM marathon pace), and below 7 they really start to feel easy to run fast in.
And if it's all placebo, that ok too as long as the PRs are real :)
The research article, link above, is clear :
it bring more if you heel strike, gains quantified by type of strike
most people using it heel strike, as the shoes is pushing to this : it is shown and discussed in the article : most peoples using it reduce their cadence for same speed, they increase stride length, so do more heel strike. Because it is easier to go fast heel striking with heavy cushion at the heel. You can't do that with racing flat.
Also clear on the non dependance on speed, between 14 and 18km/h in the study
Personnally, I get the efficiency impact tested (not precisely 4%, but significant) at 10 and 12km/h also.
And in fact, MOST peoples feel the positive impact. Especially when using heel strike.
But what many peoples also feel is the heel unstable, creating injuries.
If you do not heel strike, you have only a part of the efficiency gain (shown by the research article), but you take less injury risk (heel unstable related at least).
This is what I'm doing now. Still feeling efficiency. Less injury risk.