Technically, the Apache Royale compiler has the same capability of compiling a project from memory as ASC 2.0 (it is a fork of the same codebase). vscode-as3mxml uses the Royale compiler. I once prototyped a fast compile from memory for vscode-as3mxml, but it had limitations compared to ASC 2.0. In particular, the Royale compiler doesn't have support for embedded fonts (even when compiling for SWF onlly), which is pretty important for most Flash/AIR projects, so I didn't pursue it much further.
It's also worth mentioning the "Quick Compile and Debug" and "Quick Compile and Run" commands in vscode-as3mxml. After you've used these to compile your project once in the current session, subsequent compiles when running the same command will be faster. This is because it uses a "compiler shell" process that keeps the compiler in memory. It's not as fast as what Flash Builder can do, and I don't think it's necessarily fully incremental, but it's definitely measurably faster than running the compiler from the command line.