Explicit instantiation declaration of header only template(extern template)

I am trying to speed up the compile time of the GLM(OpenGL Mathematics). GLM makes heavy usages of C++ templates.

This is what I have tried so far.

math.h
#pragma once

#include <glm\glm.hpp>
extern template struct glm::tvec3<float, glm::highp>;

---

math.cpp
#include "math.h"
template struct glm::tvec3<float, glm::highp>;

---

And then I have three files that are using the glm::vec3 template, glm::vec3 is a typedef of glm::tvec3<float, glm::highp>. The three files a,b,c looks almost the same:

a.cpp, b.cpp, c.cpp
#include "math.h"
glm::vec3 func() {
    glm::vec3 a = glm::vec3{1,1,1};
    glm::vec3 b = glm::vec3{1,1,1};
    return a + b;
}

I am using both explicit instantiation definition and explicit instantiation declaration. So the files a,b,c should not cause implicit instantiation. But the compile time is the same as if I don´t do it.