0.5–1 keV vs. 2–5 keV lag-frequency spectrum of some of the analysed observations of MAXI J1820+070 during the rise, plateau, bright decline, and hard-soft transition phases (with colour codes and symbols as in Fig. 1). The positive lags dominating at low frequencies (≲2 Hz) are hard X-ray lags. The plot illustrates the hump-like structures characterising these low-frequency hard lags and their complex evolution throughout the first part of the outburst. At high frequencies (≳2 Hz) the (negative) soft X-ray lag becomes dominant (see inset, with the y axis in linear scale).