But you just draw the image first from vector and blit that. Vector gets all the normal bitmap speedup tricks that way.

Reading your PNG from the disk in the first place will be slower than reading a compact vector, rendering it and caching.

Re-rendering from CPU can often be faster than GPU bus access times on PCs.

Anyways, it depends on what you're optimising for. I was just illustrating how vector can be much better than bitmap in many situations.