From Signals back to Callbacks (Results)
See this Post for my motivation.
With the exchange of boost::signals2 to boost::function as delegates the project saved around 40% compilation time and 10% executable size. There are even Faster Delegates, may be we should give them a try.
Header Compile Time
Just coming from the Signals/Callback messup I took a short look of compiling (parsing, no instantiation) times of some C++ and Boost Headers I usually have always on.
On my machine (GCC 4.4, x86-64) without any other flags (Optimization did not seem to have a influence). I always took the user-time; no waiting for data or such things.
From Signals back to Callbacks
Most signals in our project are never planned to be a signal, e.g. whole communication between looslely coupled parts of a component. They could be exchanged through non-thread-safe callbacks (with boost::bind / boost::function) instead of boost::signals2' signals, as we know their instantiation time (during component init) and they usually just link to one object. Interestingly not only performances increases, also compile time seems to be heavy bound by boost::signals2. The compiler was g++ 4.4 (current Ubuntu)