In most cases maintainability wins,
  and avoiding “premature optimization”
  is very much a necessity in C++.

I agree with this conclusion. Quite often, when you start coding you don't know, where the performance bottlenecks hide, and you don't want to waste time, thinking about allocating memory for a routine which you could write equally well in any scripting language. Unfortunately plugable automatic garbage collection for less verbose performance uncritical scripting tasks within the language is not usable.

I am not familiar with the project. What are the advantages of Dgsh in comparision to pipexec: https://github.com/flonatel/pipexec

Pipexec offers a versatile pipeline construction syntax, where you specify the topology of arbitrary graphs through the numbering of pipe descriptors. Dgsh offers a declarative directed graph construction syntax and automatically connects the parts for you. Also dgsh comes with familiar tools (tee, cat, paste, grep, sort) written to support the creation of such graphs.

Modern ciphers are designed to resist chosen plaintext attacks.

Might need to read up on modern ciphers. They are protected against this kind of attack.

Probably I am wrong but there seems to be no folding support?

not even wrong :)