A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM (optical) and RXTE (X-ray) observations of the Galactic black hole binary GX339-4 in the low/hard, post-outburst state of 2007 June is presented. The optical light curves in the r',g' and u' filters show slow (~20s) quasi-periodic variability. Upon this is superposed fast flaring activity on times approaching the best time resolution probed (~50ms in r' and g') and with maximum strengths of more than twice the local mean. Power spectral analysis over ~0.004-10Hz is presented, and shows that although the average optical variability amplitude is lower than that in X-rays, the peak variability power emerges at a higher Fourier frequency in the optical. Energetically, we measure a large optical versus X-ray flux ratio, higher than that seen on previous occasions when the source was fully jet dominated. Such a large ratio cannot be easily explained with a disc alone. Studying the optical-X-ray cross-spectrum in Fourier space shows a markedly different behaviour above and below ~0.2Hz. The peak of the coherence function above this threshold is associated with a short optical time lag with respect to X-rays, also seen as the dominant feature in the time-domain cross-correlation at ~150ms. The rms energy spectrum of these fast variations is best described by distinct physical components over the optical and X-ray regimes, and also suggests a maximal irradiated disc fraction of 20 per cent around 5000Å. If the constant time delay is due to propagation of fluctuations to (or within) the jet, this is the clearest optical evidence to date of the location of this component. The low-frequency quasi-periodic oscillation is seen in the optical but not in X-rays, and is associated with a low coherence. Evidence of reprocessing emerges at the lowest Fourier frequencies, with optical lags at ~10s and strong coherence in the blue u' filter. Consistent with this, simultaneous optical spectroscopy also shows the Bowen fluorescence blend, though its emission location is unclear. However, canonical disc reprocessing cannot dominate the optical power easily, nor explain the fast variability.