This delay is almost certainly caused by Diffie-Hellman key exchange - a shared secret key is being calculated during those 70 seconds.
This should not normally take so long - we use "Enhanced DSS and Diffie-Hellman Cryptographic Provider" (part of Windows CryptoAPI) whenever available, and it's reasonably fast even on old Windows CE devices. However, some device vendors forgot to include the Diffie-Hellman CSP in their OS image (the relevant checkbox in Platform Builder used to be disabled by default), making this impossible.
As a workaround, we use a custom managed implementation of Diffie-Hellman algorithm written in C#, which is very slow. For large key sizes, the calculation can take as long as two minutes on older devices.
Please see my reply to a related support request for additional details.
The best solution would be to use an OS image with "Enhanced DSS and Diffie-Hellman Cryptographic Provider" enabled.
Alternatively, disabling Diffie-Hellman key exchange ciphers that use Diffie-Hellman with long key size would solve the issue as well, but this is strongly discouraged because short Diffie-Hellman keys are weak.
However, since Rebex File Server 2016 R3, it has been possible to use key exchange ciphers based on Elliptic Curve Diffie-Hellman (Curve 25519 and NIST P-256/P-384/P-521), which are much faster. The drawback is that they require plugins based on third-party open source libraries (see the KB article on elliptic curve support for details).