combine_polygons_below takes very long

[der-stefan]

As I'm currently preparing the OpenTopoMap vector tiles, I am playing around in depth with tilemaker's config and scripts. I tried to combine the landuse polygons (i.e. forests) on low zoom levels. However, tilemaker takes ages for this. In my case, germany-latest.osm.pbf takes ~10min when not using "combine_polygons_below" in the config. When enabled, I stopped tilemaker after ~2h.
Is this a know issue? Are you aware how the unification logic might be improved?

[systemed]

It is by definition a slow process and has been iteratively improved over tilemaker's life (e.g. #383), but that is quite an extreme slowdown and one I've not encountered before. It's always possible that there's a pathologically complex multipolygon which is boggling it.

If you could post your exact config that would help. I'd also suggest experimenting with simplification settings, because it's possible that it's not actually the combining that's the problem, it's trying to simplify the resulting polygon.