How fast can the Nano encode?

You can refer to this post:

It is to run in CBR and set vbv-size(virtual buffer size). You may tune vbv-size and check the video quality.
Another property is

  preset-level        : HW preset level for encoder
                        flags: readable, writable, changeable only in NULL or READY state
                        Enum "GstV4L2VideoEncHwPreset" Default: 1, "UltraFastPreset"
                           (0): DisablePreset    - Disable HW-Preset
                           (1): UltraFastPreset  - UltraFastPreset for high perf
                           (2): FastPreset       - FastPreset
                           (3): MediumPreset     - MediumPreset
                           (4): SlowPreset       - SlowPreset

By default it is UltraFastPreset. You may try FastPreset and MediumPreset to have better quality.