The call flow is mbedtls_hmac_drbg -> mbedtls_hmac_drbg_reseed -> mbedtls_entropy_func where the parameter, len, is 48 and MBEDTLS_ENTROPY_BLOCK_SIZE is 32. I see that mbedtls_hmac_drbg_seed sets  ctx->entropy_len = entropy_len * 3 / 2, which explains the size difference. I’ve probably mis-configured (see attached) in order to use TLS-ECDHE-RSA-WITH-AES-128-CBC-SHA, because other ciphers take too long.

Jeff Thompson  |  Senior Electrical Engineer-Firmware
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