Any modern engine does this automatically on PNG import, or as part of material/shader setup. You want different formats for different things, e.g AO, normals, bcs different formats have different compression artifacts.
How much extra time does it add for startup? For a single texture it’s probably negotiable but for a modern game, we’re talking about thousands of textures, so maybe it adds up?
i did some casual testing before of a png vs a compressed texture and its like 20x faster, so yes a big difference. most of the speedup is from not needing to calculate mipmaps since they are already precalculated.