Performance regression in v4.0 with OpenStreetMap Carto?

Tile rendering on OpenStreetMap servers has gotten slower after an upgrade from Debian 12 (Mapnik v3.1) to Debian 13 (Mapnik v4.0), see [openstreetmap/operations#1375](https://github.com/openstreetmap/operations/issues/1375) (also [openstreetmap-carto#5276](https://github.com/openstreetmap-carto/openstreetmap-carto/issues/5276)).

A few sample numbers from just looking at the data of some monitoring graphs:
- ~46% increased low zoom (0-12) rendering job duration (92 min -> 134 min) on server culebre ([Time by Zoom](https://prometheus.openstreetmap.org/d/wyyzhZKMk/tile-rendering?orgId=1&from=2026-05-08T22:00:00.000Z&to=2026-05-10T02:00:00.000Z&timezone=utc&var-instance=culebre&viewPanel=panel-36)) ¹
- ~48% increased low zoom (0-12) rendering job duration (175 min -> 259 min) on server odin ([Time by Zoom](https://prometheus.openstreetmap.org/d/wyyzhZKMk/tile-rendering?orgId=1&from=2026-05-08T21:00:00.000Z&to=2026-05-10T02:00:00.000Z&timezone=utc&var-instance=odin&viewPanel=panel-36)) ¹
- ~129% increased [time per tile](https://prometheus.openstreetmap.org/d/wyyzhZKMk/tile-rendering?orgId=1&from=2026-04-18T09:25:00.000Z&to=2026-05-30T09:25:00.000Z&timezone=utc&var-instance=culebre&viewPanel=panel-49) for zoom 13 (5.9s -> 13.5s avg) on server culebre ²:
  <img width="500" alt="screenshot time per tile, z13, culebre" src="https://github.com/user-attachments/assets/06f3cb6d-96de-44a5-b9d7-6fcdde154781" />

As I understand, the regression in #4521 does not apply here, as `srs` is not an uppercase EPSG code, right?
  `srs: &srs "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over"`  ([openstreetmap-carto/project.mml](https://github.com/openstreetmap-carto/openstreetmap-carto/blob/v6.0.0/project.mml#L4))

As the Mapnik upgrade was part of a Debian upgrade I'm not really sure if this is actually a Mapnik regression, although it seems likely. Maybe you have some ways to reproduce this issue?

*[disclaimer: I'm just a messenger, neither a member of the OSMF Operations Working Group nor an openstreetmap-carto contributor]*

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¹ *time between first entry for z0 and last entry for z12*
² *average of values before and after 2026-05-09 09:30*

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