diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index be4e28f9..4d1ac13d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -8,6 +8,13 @@
 # Tests are built once and run in 4 shards. A few heavy tests (see .config/nextest.toml) are
 # skipped on pull requests and run on pushes to main and nightly.
 # Cross-compilation is opt-in through Run workflow; release builds remain in release.yml.
+# Pushes to main and merge-queue groups use both pools: Clippy, the doc tests and test shards
+# 1-2 run on the self-hosted runner (`self-hosted-fast`, warm build directories via
+# scripts/ci/warm-target.sh), everything else on GitHub's runners. The test archive is built on
+# GitHub's runners: the self-hosted machine has a newer glibc, so its binaries wouldn't start
+# on ubuntu-24.04, while the archive runs there unchanged (same /home/runner/work checkout
+# path). Pull requests, the nightly run and Run workflow use GitHub's runners only; the
+# self-hosted runner refuses any job that isn't a push to main or a merge-queue group.
 name: CI
 
 on:
@@ -134,14 +141,19 @@ jobs:
     name: Native Linux Clippy
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ubuntu-24.04
+    runs-on: ${{ (github.event_name == 'merge_group' || (github.event_name == 'push' && github.ref == 'refs/heads/main')) && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Warm build directory (self-hosted runner)
+        if: runner.environment == 'self-hosted'
+        run: scripts/ci/warm-target.sh clippy
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy
-      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+      # The self-hosted runner keeps its build directory and Cargo home instead.
+      - if: runner.environment == 'github-hosted'
+        uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
         with:
           key: native-clippy
       - name: Clippy
@@ -200,7 +212,7 @@ jobs:
   tests:
     name: Native Linux tests (${{ matrix.shard }}/4)
     needs: [changes, build-tests]
-    runs-on: ubuntu-24.04
+    runs-on: ${{ ((github.event_name == 'merge_group' || (github.event_name == 'push' && github.ref == 'refs/heads/main')) && matrix.shard <= 2) && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 60
     strategy:
       fail-fast: false
@@ -356,7 +368,7 @@ jobs:
     name: Doc tests
     needs: changes
     if: needs.changes.outputs.native == 'true'
-    runs-on: ubuntu-24.04
+    runs-on: ${{ (github.event_name == 'merge_group' || (github.event_name == 'push' && github.ref == 'refs/heads/main')) && 'self-hosted-fast' || 'ubuntu-24.04' }}
     timeout-minutes: 45
     env:
       CARGO_INCREMENTAL: "0"
@@ -365,8 +377,13 @@ jobs:
       SCCACHE_IGNORE_SERVER_IO_ERROR: "1"
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - name: Warm build directory (self-hosted runner)
+        if: runner.environment == 'self-hosted'
+        run: scripts/ci/warm-target.sh doc-tests
       - uses: dtolnay/rust-toolchain@stable
-      - uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
+      # The self-hosted runner keeps its build directory and Cargo home instead.
+      - if: runner.environment == 'github-hosted'
+        uses: Swatinem/rust-cache@6323deb102c322ba6fcbdcafc7e3dddab59af2b6 # v2.9.2
         with:
           key: doc-tests
       - name: Set up compiler cache
diff --git a/scripts/ci/warm-target.sh b/scripts/ci/warm-target.sh
new file mode 100755
index 00000000..6bbb3aed
--- /dev/null
+++ b/scripts/ci/warm-target.sh
@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Self-hosted runner only: keep a job's Cargo build directory between runs, outside the
+# checkout (actions/checkout's clean would delete it), and point CARGO_TARGET_DIR at it for the
+# rest of the job. Usage: warm-target.sh NAME
+# Each self-hosted runner has its own /home/runner, so two never share a directory.
+set -euo pipefail
+name=${1:?usage: warm-target.sh NAME}
+dir=/home/runner/work/cargo-target/$name
+# Start over when it has grown past 40 GB: old dependency versions pile up.
+if [ -d "$dir" ] && [ "$(du -sm "$dir" | cut -f1)" -gt 40000 ]; then
+  echo "warm-target: $dir is over 40 GB, starting a fresh build"
+  rm -rf "$dir"
+fi
+mkdir -p "$dir"
+echo "CARGO_TARGET_DIR=$dir" >> "$GITHUB_ENV"
+echo "warm-target: $dir ($(du -sh "$dir" | cut -f1))"