It isn't practical on low end IoT devices which are now more than ever having to move toward encrypted communications. I work on a ~160Mhz product that typically takes 5+ minutes to generate a 2048-bit RSA key but can craft a 256-bit ECC key in 1 sec. There are no hardware resources that can speed this up. Stronger RSA is a dead end. It also chews up a non-trivial fraction of available RAM.
If I may ask, what SSH implementation are you using? One of the libssh's? It's always a pain when one has to forego OpenSSH because it just won't run on the low-end devices...
