Large number of CanvasItems in the scene slow down input

[TrickMe]

Godot version

3.4.2 stable official

System information

Linux Mint 20.2 Cinnamon, Kernel 5.11, GLES3, AMD Athlon II X4, AMD Radeon Rx 570X, Mesa 21.3

Issue description

Mouse movement cripples performance massively if many (~10.000) CanvasItems (Control or Node2D) are in the scene.
It's specially bad if Input.set_use_accumulated_input() is false, but is also noticeable if true.

Steps to reproduce

Place many CanvasItems in the scene, about 10.000 and move the mouse around.

Minimal reproduction project

rep_manyLines.zip

[HubbleCommand]

Win10 4th gen intel i7, so slightly better than @TrickMe.

Just did a simple test where I spawned 10k sprites & 10k buttons. No input lag.

For fun I made it spawn 50k of each. The mouse was fine, but there was a slight delay in the "pressed" signals from the buttons.

[TrickMe]

@HubbleCommand
A project of mine is also not effected by this issue, although it uses thousands of sprites.
Have you checked with my minimum reproduction project?
Does spawning even more sprites have any effect?

[HubbleCommand]

I will look at the project later if I have time.

There are 2 solutions if you have that many sprites that are affecting performance:

If not all sprites are on the screen, and you have large / high detail sprites, you can convert them to 2D meshes.
If pretty much (preferably all) sprites are on the screen, you can use MultiMesh2D.

While multimesh2D uses 2D meshes, you use them in opposite scenarios:

• Mesh2D splits a sprite into parts, and will only draw the parts that are on the screen
• MultiMesh2D will always draw all of the sprites and all their parts (if it works like the 3D version), but reduces memory as they all use the same resource

[ConteZero]

I can confirm the issue, tested on Linux with an Intel i5 11500, here some results:
I get 1 fps when moving the mouse with 100.000 Node2D
I get 1 fps when moving the mouse with 50.000 PanelContainers
I get 1 fps when moving the mouse with 15.000 PanelContainers with a Label child

[Calinou]

I'd look into using a C++ profiler to determine where the bottleneck is.

That said, asking the engine to perform 15,000+ input events every time the mouse cursor is moved is asking a lot. It'd probably be better to throttle the frequency at which InputEventMouseMotions are sent if the input system is overloaded.

[ConteZero]

For the PanelContainer test I've added the nodes as a child of a VBoxContainer so only a couple of them are visible on screen (18 out of 15.000 are visible).
Probably limiting the mouse motion check to just the nodes on the screen would help in this case.

[ConteZero]

I've also tried to use MOUSE_FILTER_IGNORE but it doesn't solve the issue.
It could be useful to have a real "ignore" mouse filter that will ignore the event (as if the control was hidden):
godotengine/godot-proposals#3613

[ConteZero]

I've done the same test with a low spec pc (Intel Celeron J3355):
I get 1 fps when moving the mouse with 10.000 Node2D
I get 1 fps when moving the mouse with 6.000 PanelContainers
I get 1 fps when moving the mouse with 4.000 PanelContainers with a Label child