There's also the info-manual, but I'm doubting it offers any more detail than the manpage. They're both probably equally complete.
What slows you down is when new ideas are introduced, like when you go from multi-paradigm languages like Python & Typescript to a more purely functional language like Haskell, or learning about ownership in Rust where there may be no equivalent in other languages. But ideally you only need to learn them once, and in a way that allows you to identify and apply them in a variety of contexts.
But a professional C++ programmer would have designed their program a lot differently than the professional Java programmer. Simple syntax translation isn't good enough here.
Programming is easy, learning the fancy tricks is the hard part.
Grab your variable declarations, inheritance support, static typign, flow control, loop forms, your variable casting, your i/o for files and the standard output for strings and you can learn any language pretty easily.
Edit: String concatination if it's not Java-like (because . concatination is a thing)