diff --git a/.github/actions/checkout/action.yml b/.github/actions/checkout/action.yml
index a07edb097d7b9..1b61b9b0d08d0 100644
--- a/.github/actions/checkout/action.yml
+++ b/.github/actions/checkout/action.yml
@@ -13,39 +13,88 @@ inputs:
 runs:
   using: composite
   steps:
-    - if: inputs.merged-as-untrusted-at
-      # Would be great to do the checkouts in git worktrees of the existing spare checkout instead,
-      # but Nix is broken with them:
-      # https://github.com/NixOS/nix/issues/6073
-      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-      with:
-        ref: ${{ inputs.merged-as-untrusted-at }}
-        path: untrusted
-
-    - if: inputs.target-as-trusted-at
-      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-      with:
-        ref: ${{ inputs.target-as-trusted-at }}
-        path: trusted
-
-    - if: inputs.pinned-from
-      id: pinned
-      uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+    - uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
       env:
+        MERGED_SHA: ${{ inputs.merged-as-untrusted-at }}
         PINNED_FROM: ${{ inputs.pinned-from }}
+        TARGET_SHA: ${{ inputs.target-as-trusted-at }}
       with:
         script: |
-          const path = require('node:path')
-          const pinned = require(path.resolve(path.join(process.env.PINNED_FROM, 'ci', 'pinned.json')))
-          core.setOutput('pinned-at', pinned.pins.nixpkgs.revision)
+          const { spawn } = require('node:child_process')
+          const { join } = require('node:path')
 
-    - if: steps.pinned.outputs.pinned-at
-      uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-      with:
-        ref: ${{ steps.pinned.outputs.pinned-at }}
-        path: pinned
-        sparse-checkout: |
-          lib
-          maintainers
-          nixos/lib
-          pkgs
+          async function run(cmd, ...args) {
+            return new Promise((resolve, reject) => {
+              const proc = spawn(cmd, args, {
+                stdio: 'inherit'
+              })
+              proc.on('close', (code) => {
+                if (code === 0) resolve()
+                else reject(code)
+              })
+            })
+          }
+
+          // These are set automatically by the spare checkout for .github/actions.
+          // Undo them, otherwise git fetch below will not do anything.
+          await run('git', 'config', 'unset', 'remote.origin.promisor')
+          await run('git', 'config', 'unset', 'remote.origin.partialclonefilter')
+
+          // Getting the pinned SHA via API allows us to do one single fetch call for all commits.
+          // Otherwise we would have to fetch merged/target first, read pinned, fetch again.
+          // A single fetch call comes with a lot less overhead. The fetch takes essentially the
+          // same time no matter whether its 1, 2 or 3 commits at once.
+          async function getPinnedSha(ref) {
+            const { content, encoding } = (await github.rest.repos.getContent({
+              ...context.repo,
+              path: 'ci/pinned.json',
+              ref,
+            })).data
+            const pinned = JSON.parse(Buffer.from(content, encoding).toString())
+            return pinned.pins.nixpkgs.revision
+          }
+
+          const commits = [
+            {
+              sha: process.env.MERGED_SHA,
+              path: 'untrusted',
+            },
+            {
+              sha: process.env.PINNED_FROM === 'untrusted' && (await getPinnedSha(process.env.MERGED_SHA)),
+              path: 'pinned'
+            },
+            {
+              sha: process.env.TARGET_SHA,
+              path: 'trusted',
+            },
+            {
+              sha: process.env.PINNED_FROM === 'trusted' && (await getPinnedSha(process.env.TARGET_SHA)),
+              path: 'pinned'
+            }
+          ].filter(({ sha }) => Boolean(sha))
+
+          console.log('Checking out the following commits:', commits)
+
+          // Fetching all commits at once is much faster than doing multiple checkouts.
+          // This would fail without --refetch, because the we had a partial clone before, but changed it above.
+          await run('git', 'fetch', '--depth=1', '--refetch', 'origin', ...(commits.map(({ sha }) => sha)))
+
+          // Create all worktrees in parallel.
+          await Promise.all(commits.map(async ({ sha, path }) => {
+            // Checking out onto tmpfs takes 1s and is faster by at least factor 10x.
+            await run('mkdir', path)
+            switch (process.env.RUNNER_OS) {
+              case 'macOS':
+                await run('sudo', 'mount_tmpfs', path)
+                // macOS creates this hidden folder to log file system activity.
+                // This trips up git when adding a worktree below, because the target folder is not empty.
+                await run('sudo', 'rm', '-rf', join(path, '.fseventsd'))
+                break
+              case 'Linux':
+                await run('sudo', 'mount', '-t', 'tmpfs', 'tmpfs', path)
+                break
+            }
+            await run('git', 'worktree', 'add', path, sha, '--no-checkout', '--force')
+            await run('git', '-C', path, 'sparse-checkout', 'disable')
+            await run('git', '-C', path, 'checkout', '--progress')
+          }))
diff --git a/.github/workflows/eval.yml b/.github/workflows/eval.yml
index 0b70521d62b3a..805303ac679e7 100644
--- a/.github/workflows/eval.yml
+++ b/.github/workflows/eval.yml
@@ -217,6 +217,7 @@ jobs:
       - name: Check out the PR at the target commit
         uses: ./.github/actions/checkout
         with:
+          merged-as-untrusted-at: ${{ inputs.mergedSha }}
           target-as-trusted-at: ${{ inputs.targetSha }}
           pinned-from: trusted
 
@@ -240,7 +241,6 @@ jobs:
         env:
           AUTHOR_ID: ${{ github.event.pull_request.user.id }}
         run: |
-          git -C trusted fetch --depth 1 origin ${{ inputs.mergedSha }}
           git -C trusted diff --name-only ${{ inputs.mergedSha }} \
             | jq --raw-input --slurp 'split("\n")[:-1]' > touched-files.json
 
@@ -392,7 +392,7 @@ jobs:
         uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31
 
       - name: Ensure flake outputs on all systems still evaluate
-        run: nix flake check --all-systems --no-build ./untrusted
+        run: nix flake check --all-systems --no-build './untrusted?shallow=1'
 
       - name: Query nixpkgs with aliases enabled to check for basic syntax errors
         run: |