diff --git a/lib/maptool/pmtiles_builder.rb b/lib/maptool/pmtiles_builder.rb
index cccf2e9..de586e8 100644
--- a/lib/maptool/pmtiles_builder.rb
+++ b/lib/maptool/pmtiles_builder.rb
@@ -4,6 +4,7 @@ require "uri"
 require "json"
 require "tmpdir"
 require "fileutils"
+require "etc"
 
 module Maptool
   # Regenerates data/offline.pmtiles and data/terrain.pmtiles for the
@@ -276,7 +277,12 @@ module Maptool
       # this process's working directory (the app root, so in practice
       # that's data_dir's own "sources" subdirectory) — fetched once,
       # reused by every later build.
-      run_planetiler!("--osm-path=#{osm_path}", "--output=#{tmp_path}", "--download")
+      # --threads: Planetiler's own core-count autodetection can come up
+      # short in a container (cgroup/JVM detection quirks vary by runtime),
+      # leaving most of a many-core host idle — Etc.nprocessors asks the
+      # OS directly (sched_getaffinity on Linux), which correctly reflects
+      # whatever's actually usable by this process even under a container.
+      run_planetiler!("--osm-path=#{osm_path}", "--output=#{tmp_path}", "--download", "--threads=#{Etc.nprocessors}")
       File.rename(tmp_path, File.join(data_dir, "offline.pmtiles"))
       File.write(offline_meta_path(data_dir), JSON.generate(sources: sources.map { |s| s.slice("code", "slug", "last_modified") }))
     ensure