Why it took 28s to set up the skybox

Mainly because CubeMapToSphericalPolynomialTools.ConvertCubeMapTextureToSphericalPolynomial took 28s, almost all of which are calling CubeMapToSphericalPolynomialTools.ConvertCubeMapToSphericalPolynomial

This is my code

  const cubeTexture = new CubeTexture(
            "/static/skyboxes/cloudless/TropicalSunnyDay",
            this._scene
          );
          this._skyboxMesh = this._scene.createDefaultSkybox(
            cubeTexture,
            true,
            SKYBOX_SIZE
          );

Just wondering how big the image / texture is?

500kb

This is a really heavy operation when using pbr and if you do not use it for PBR there is no need to create the polynomials. And in case you do, you should convert offline to a .env file which already contains the required info: Using An HDR Environment For PBR | Babylon.js Documentation

The size asked is more the width of each faces ?