Using TIME in canvas_item shader drops FPS by half

[vigreco]

Godot version:
master (91adb58)

OS/device including version:
OS X 10.11.6
NVIDIA GeForce 9400M

Issue description:
Using TIME, also in a very simple shader, causes a very noticeable performance degradation.
On my machine FPS drop from 60 to 20 (sometimes 30) with only one sprite in the scene.

Steps to reproduce:
Use TIME in a shader. Also only assigning TIME to a variable generates the issue:

shader_type canvas_item;

void fragment() {
    float t = TIME;
}

Not sure if still related, but Godot often crashes when I use TIME dependent shaders. Same as #9123.

[reduz]

Using TIME causes the editor to refresh in the object is visible, this may be your source of degratation.
I don't really think this is a bug.

[reduz]

Also the editor FPS are probably not a good reference. I will close this unless more information is provided.