Thanks for the report! Always appreciated.
The tiles might look small, but they're all 256x256 pixels. Still, it only loads ~5.4MB in total though. I don't think that's so big. Takes about 4s in incognito mode for me.
The large number of small files could be a problem, depending on the how the browser requests them. I need to figure out how to bundle them together, but worried that might take a while to figure out.
Compressing the PNGs and minifying the JS cuts the file size in half and we'll see if that's any better.