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

A Black Hole X-ray Binary at $\sim$100 Hz: Multiwavelength Timing of MAXI J1820+070 with HiPERCAM and NICER