I talked to the developer of the Godot Vita port to find out why the load times were so long. Basically, Godot’s GLES2 renderer doesn’t have a shader cache, while GLES3 does. But we don’t have GLES3 on the Vita so it has to compile shaders at runtime every single time. Since Godot is open source someone could probably implement a shader cache (not me though since I barely know C and I certainly don’t know anything about GL shaders or game engines), but since it only affects the Vita I honestly doubt it.
tl;dr: it’s unfortunately something I can’t fix