Performance on what? If you are talking about frames per second, there are mainly two impacting factors you can tweak: CPU and GPU.
CPU usage can be reduced by making your game perform less calculations:
Choose appropriate nodes, choose the correct data structures (array? dictionary?), use fast algorithms, know what's fast and slow in the language you use, prefer signals and events instead of
_process, use servers directly if nodes have too much overhead etc...
GPU usage can be reduced by making your graphics card do less work:
Draw less pixels, draw less geometry, don't stack too many elements on screen, don't stall the GPU by getting data from it, write efficient shader code (that one also depends on which platform you run on).
Optimizing your game depends on which of these two you are bound to. It's not always due to scripts, and it doesn't always mean you will need C++. It's down to knowing what things cost (which you can learn by reading docs and by experience).
If your game has performance problems, a good starting point is to have a look at the profiler tab of the editor, while you are debugging it, to see which part takes more time.
(and as always, remember it will always run a bit slower in editor than once exported).
As for optimization tutorials... it's a wide topic that can become very specific at the same time, so you may find lots of things with a web search.
So yeah... I don't know a general optimization tutorial on Godot, but if you have a more specific example showing slowness I might be able to explain more what can be done.