Stellar flares are sudden brightenings caused by magnetic reconnection and are frequently observed on late-type stars. High-cadence photometry of flares provides valuable insights into the mechanisms of these events, yet such observations remain scarce. We seek to explore the sub-second fine structure of stellar flares and assess the information content in high-speed photometry. New 0.3 s-cadence photometry from a six-year-long observing campaign of the active M-dwarf AD Leo is presented. We use time--frequency analysis to detect quasi-periodic pulsations in the decay phase of flares. We explore statistical measures of time series complexity of the detected flares to quantify the information gain achievable with high-cadence photometry. We detect 42 flares in 211 hours of observations. The flare frequency distribution is consistent with the previous literature. We find no quasi-periodic pulsations with periods below a few seconds, and identify two candidate signals with periods around 1 and 3 min. Using different measures of complexity on the binned flare light curves we confirm the advantages of high observing cadence. However, we also find a plateau up to a binning of ~4--5 s for a few complex flares, suggesting that an exposure time of a few seconds is usually enough to retain most of the information carried by a single-filter observation. New photometric observations of AD Leo revealed sub-structures of flare light curves on the timescale of a few seconds, but we found no features on timescales below that.