I agree with your general point that most companies/projects do a terrible job optimizing for slow computers/networks, but OpenSSH is from the OpenBSD people, who are well-known for supporting ancient hardware [0]. Picking a random architecture, they fully support a system with only 64MB of memory [1], and the base install includes SSH. So I suspect that OpenSSH is fairly well tested on crappy computers/networks.

[0]: https://www.openbsd.org/plat.html

[1]: https://www.openbsd.org/landisk.html#hardware