Based on a user comment on the new map, I have added another zoom level to the Toronto transit map. I didn’t do this before because Illustrator wouldn’t let me export a large enough image to PNG, in this case 13794 by 8192. Instead I exported the image in two parts and collated the resulting images using ImageMagick. The detail on that zoom level is high enough so that the names of side streets are now visible.
In addition to adding the higher zoom level, I also re-rendered the tiles for levels 3 and 5 as the scale on the line strokes was off. Surprisingly, this resulted in a 50% bandwidth savings for the level 3 tiles so they should download a little faster now.
To further increase download speed, I have spread the tiles over four domains based on the recommendations of a study on optimising page load time. Incidentally, Google’s map tiles are also spread out over four domains and I coincidentally chose the same simple algorithm.