  We report on simultaneous sub-second optical and X-ray timing observations of
the low mass X-ray binary black hole candidate MAXI J1820+070. The bright 2018
outburst rise allowed simultaneous photometry in five optical bands ($ugriz_s$)
with HiPERCAM/GTC (Optical) at frame rates over 100 Hz, together with NICER/ISS
observations (X-rays). Intense (factor of two) red flaring activity in the
optical is seen over a broad range of timescales down to $\sim$10 ms.
Cross-correlating the bands reveals a prominent anti-correlation on timescales
of $\sim$seconds, and a narrow sub-second correlation at a lag of $\approx$+165
ms (optical lagging X-rays). This lag increases with optical wavelength, and is
approximately constant over Fourier frequencies of $\sim$0.3-10 Hz. These
features are consistent with an origin in the inner accretion flow and jet base
within $\sim$5000 Gravitational radii. An additional $\sim$+5 s lag feature may
be ascribable to disc reprocessing. MAXI J1820+070 is the third black hole
transient to display a clear $\sim$0.1s optical lag, which may be common
feature in such objects. The sub-second lag $variation$ with wavelength is
novel, and may allow constraints on internal shock jet stratification models.
