Skybox shader visual artifacts

I created a shader that exists on the inside of a dome with the intention of making a custom skybox. However, something about the shader is not optimized, so it updates very slowly, looks very low resolution, and results in some very bad visual bugs (circled in the image).

My question is - when it comes to using a shader for a skybox, what specifically is causing the performance issues? I’m thinking it could be the efficiency of the shader itself, the number of verts in the sphere, the fact that i’m using highp floats or maybe just the number of pixels that the shader has to write. Here’s a link to the shader if that helps: cloudy-sky-shader.ts · GitHub