Fast-mode is likely your best choice for short cycle time and high waveform detail at a T/Div of 5ms. Cycle time will then be close to 100ms for NORM and slightly above 100ms for AUTO. This mode acquires a 10x oversampled signal and use simple averaging for display rendering.
The Nano does not have the capacity to do significant processing on real-time waveform data when high-speed data acquisition is ongoing. Part of the reason is that DMA (used for acquisition) steels bus-cycles and prevents concurrent data access. Processing is limited to searching for a trigger and so display data is scaled and refreshed directly from raw buffer data between cycles. If you stop capture in fast-mode and use the zoom capability, you should see progressively more waveform detail. Measurements and XML export data are also obtained directly from the acquisition buffer in order to preserve full ADC range. You should be able to trigger on waveform details in fast-mode even though you may not see such details on the display (due to averaging). This is also why you see a somewhat flattened signal when you compare fast-mode to normal in real-time for the same waveform (see page 5 of this thread for more detail on this issue).
What we could do is add an optional min/max (peak) mode for display rendering of fast-mode acquisitions and this may be something to consider for a future upgrade.