You mean the size of the Defold editor downloaded from here: Download Defold? The reason that I’m asking is that when you write “engine” I mainly think of the the runtime part which gets installed with your game content. Anyway, yes, the editor and tools would decrease in size if we didn’t have to package the bundlers for each platform and instead let users download the bundlers as plugins. It would also to some extent complicate things and it is not something we plan to do at the moment.
That is of course a risk, but such a thing would quickly be spotted by the users and hopefully the extension author would then address the performance issues and fix them. The extension system itself is designed to be performant, but there is always a little bit of overhead each time you call from Lua into native code. If you call your extension code thousands of times per frame it will be noticeable.
It doesn’t really have anything to do with if your game is 2D or 3D. I don’t see any problems with using Lua for game logic in a 3D game. The engine is still written in C++ and will give you enough performance regardless if it is 2D or 3D. But yes, if you end up in a situation where you can’t get enough performance from Lua, then sure, move that piece of performance critical code to an extension and use C++ instead!