Can you give practical examples about cases, where Lua code could be optimized?
No I couldn't. That's a specialist's subject, I'm not one of them, and my favorite way to optimize is to write whatever must go really fast in C. Which is one of the reasons why I love Lua.
replacing a number of a= a..b with table.concat() is way outside of the optimization scope, right?
That kind of macro optimization could be implemented with some term rewriting system, which is a particular case of AST manipulation. There are interesting things to do at high level, look at
http://mlton.org for instance, but that would be a big project to undertake, for a limited benefit given Lua's C-friendliness.
