  We report on a multi-epoch campaign of rapid optical/X-ray timing
observations of the superbright 2018 outburst of MAXI J1820+070, a black hole
low-mass X-ray binary system. The observations spanned 80 days in the initial
hard-state, and were taken with NTT/ULTRACAM and GTC/HiPERCAM in the optical
(ugriz filters at time resolutions of 8--300 Hz) and with ISS/NICER in X-rays.
We find (i) a growing anti-correlation between the optical and X-ray
lightcurves, (ii) a steady, positive correlation at an optical lag of 0.2 s
(with a longer lag at longer wavelengths) present in all epochs, and (iii) a
curious positive correlation at \textit{negative} optical lags in the last,
X-ray softest epoch, with longer wavelengths showing a greater correlation and
a more negative lag. To explain these we postulate the possible existence of
two synchrotron-emitting components; a compact jet and a hot flow. In our
model, the significance of the jet decreases over the outburst, while the hot
flow remains static (thus, relatively, increasing in significance). We also
discuss a previously discovered quasi-periodic oscillation and note how it
creates coherent optical time lags, stronger at longer wavelengths, during at
least two epochs.
