GNOME Maps and the tile problem

The data does need to be structured properly. One trick for fast vector rendering I recall reading about is to make sure long lines have points even where they don't need them in order to guarantee at least one point in each visible area of the space partition tree. And I think those extra points can be dynamically generated from the previous zoom level. Otherwise your code is back at calculating all kinds of extra line intersections just to see if it's visible or not. And of course priority levels so that tiny features aren't being included when zoomed far out.

I suspect that, like much modern software, lots of programmers just go for brute force instead of cleverness. "Let's just draw EVERY line and find its intersection with the view! Yeah! And instead of a regular BSP with an insanely fast binary search we'll have these local object bundles with irregular edges (eg, the state of Maryland) and we'll run a view intersection test against every bundle to see if we should be drawing those." Etc.