Each use of mxmlc is very slow because the whole code is recompiled. The more code the more time it will take. It would be perfect if the compilator took into process only changes. It is possible and special tool comes with rescue. In bin directory of Flex SDK you can find fcsh – it is shell which you can use to incremental compilation of your ActionScript code.
Despite appearances it’s very easy. All you need is Flex SDK which is available for download on Adobe site. There are sufficient tools in this SDK to compile ActionScript 3 code into single SWF file. Furthermore there is no problem to embed resource files (MP3, PNG) with it. I assume that you have Linux operating system and you are familiar with using the terminal. Obviously you have to know ActionScript 3 basics at least.
Compile ActionScript with mxmlc is very slow, because each time the whole code is recompiled. Flex SDK provide tool fcsh shell which can speed up this process by incremental compilation. Unfortunately usage of that shell is inconvinient. Mainly by need to type each time compile 1 command to recompile changed part of code (arrow up to repeat last command doesn’t work). I wrote a script that helps in usage of that shell.