PSK cipher suites are a superb choice in low resource environments where both ends of the connection can be controlled. With PSK, each side of the connection has an already agreed upon key to use rather than agreeing on one during the TLS handshake. This reduces resource consumption for each session using PSK.
For example, on one of wolfSSL’s test machines the cipher suite DHE-PSK-AES128-CBC-SHA256 has an average connection time of 3.498 milliseconds with a peak byte usage of 6,335. On the same machine a similar cipher suite DHE-RSA-AES128-SHA256, not using PSK, has an average connection time of 7.146 milliseconds and peak byte usage of 19,431. wolfSSL always recommends using ephemeral keys (DHE or ECDHE) to maintain forward secrecy but in an ultra limited resource environment, memory and speed can be further improved by using a static PSK cipher suite such as PSK-AES128-CBC-SHA.
In addition to RAM reduction, using PSK can reduce the library footprint size as well. One of the smallest wolfSSL builds to date has been the LeanPSK build, which comes in at around 21kB. For comparison, a typical build on an embedded, optimized compiler will be 60-100kB.