Issue 9853 reproduction, BEFORE
Commit: ed9b7ca9c5
Linux; git version 2.43.0; Node 24.21.0
Real app: vp run dev; web localhost:6005; isolated .t3 state.
Disposable fixture: superproject with initialized module, 16 MiB random blob, local git:// remote.
Browser steps: Add project /tmp/issue-9853-fixture/super; New worktree; From main (remote); first message Reply OK only. Do not edit any files.
Source and destination module HEAD: 26b92b8e3048f6fc7db3096c1723b2c2e23d7682
Destination fresh pack: 16782544 bytes.
Observed repeated clone/index-pack; not a claim of universal speedup or reproduction of 18-second latency.

Git Trace2 start records:
2026-10-22T00:43:50.207150Z ['git', '--git-dir', '/tmp/issue-9853-fixture/super/.git', 'worktree', 'list', '--porcelain', '-z']
2026-10-22T00:44:00.696217Z ['git', '-c', 'checkout.workers=0', 'worktree', 'add', '-b', 't3/1e65db2f', '--', '/home/ubuntu/wt/issue-9853/.t3/worktrees/super/t3-1e65db2f', 'fffb9139554e10f74ad3132fbdfcbea22f2abb1d']
2026-10-22T00:44:00.757674Z ['git', 'submodule', 'update', '--init', '--recursive']
2026-10-22T00:44:00.893844Z ['/usr/lib/git-core/git', 'index-pack', '--stdin', '--fix-thin', '--keep=fetch-pack 148069 on fs-36', '--check-self-contained-and-connected']
2026-10-22T00:44:01.886630Z ['git', '--git-dir', '/tmp/issue-9853-fixture/super/.git', 'worktree', 'list', '--porcelain', '-z']
2026-10-22T00:44:08.854091Z ['git', '--git-dir', '/tmp/issue-9853-fixture/super/.git', 'worktree', 'list', '--porcelain', '-z']

Server spans:
GitVcsDriver.fetchRemote 25.99864 ms {'_tag': 'Success'}
fetchRemote 26.567176 ms {'_tag': 'Success'}
GitVcsDriver.createWorktree 98.747212 ms {'_tag': 'Success'}
GitVcsDriver.createWorktree.updateSubmodules 569.469188 ms {'_tag': 'Success'}
GitVcsDriver.createWorktree.configureBaseRef 11.074396 ms {'_tag': 'Success'}
GitVcsDriver.createWorktree.configureComparisonRef 11.883162 ms {'_tag': 'Success'}
createWorktree 778.120593 ms {'_tag': 'Success'}
GitManager.createWorktree 816.849043 ms {'_tag': 'Success'}