Interesting paper in press at the International Journal of Sports Physiology and Performance, showing that the slower you are, the less benefit you get from carbon shoes (at least in terms of efficiency; some folks like them for the reduced muscle damage).
"Purpose: The Nike Vaporfly line of running shoes improves running economy by ∼2.7% to 4.2% at running speeds of 13 to 18 km·h−1. It is unclear whether similar benefits are conferred at slower speeds. Our purpose was to determine the effects of the Nike ZoomX Vaporfly Next% 2 (VFN2) on running economy at 10 and 12 km·h−1 compared with a mass-matched control (CTRL) shoe. Methods: Sixteen runners completed 4 × 5-minute trials at both 10 and 12 km·h−1 on the same day. Each shoe was tested twice at each speed in a counterbalanced, mirrored sequence. Data are displayed as mean (SD). Results: A 2-way repeated-measures analysis of variance showed a significant shoe × speed interaction for oxygen consumption (P = .021). At 12 km·h−1, oxygen consumption (in mL·kg−1·min−1) was lower (−1.4% [1.1%]; P < .001) for VFN2 (35.8 [1.7]) relative to CTRL (36.4 [1.7]). That was greater in magnitude than the differences observed at 10 km·h−1 (−0.9% [1.8%]; P = .065) between VFN2 (29.4 [1.9]) and CTRL (29.6 [1.9]). Conclusions: From these data, it appears that the VFN2 still enhances running economy at 10 and 12 km·h−1; however, these benefits are smaller in magnitude compared with previous research at faster speeds."