PNG files image popping

I’m noticing that PNG images are popping in quite late. In the network tab I can see that they take 300ms to load for 6kb files and they aren’t getting cached.

I also have SVG files of 6kb on the same page and they never pop in late. The network tab says they are in the memory cache and take 0ms to load.

What’s going on here? Why are PNG files treated differently?