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I recently discovered that dropbear, compared to openssh over same link
with same host and user keys, transfers files several times slower -
sometimes up to an order of magnitude slower. This equally applies to
both fast LAN IPv4 networks (tried on 1Gbps) and relatively slow "over
re6stnet" networks with geographically separated client and server.

The problem turned out to be dropbear has very small receive window size
default:

    https://github.com/mkj/dropbear/blob/DROPBEAR_0.53.1/options.h#L258

    ---- 8< ----
    /* Window size limits. These tend to be a trade-off between memory
       usage and network performance: */
    /* Size of the network receive window. This amount of memory is allocated
       as a per-channel receive buffer. Increasing this value can make a
       significant difference to network performance. 24kB was empirically
       chosen for a 100mbit ethernet network. The value can...