codex - 💡(How to fix) Fix Desktop performance collapses in profiles with a few very large local conversation histories (typing, scrolling, thread list, random exits)

Summary: This is broader than slow thread switching. In profiles that contain a small number of extremely large local conversation histories, Codex desktop becomes slow across multiple surfaces: typing, scrolling inside a conversation, scrolling the thread list, delayed UI updates, slow thread switching, and occasional app exits. My strongest verified signal is not total conversation count, but the presence of a few giant local histories. Some of those heavy histories also had recurring thread heartbeat automations attached, which may amplify the problem, but that variable is not yet fully isolated.

Environment:

• Codex desktop app 26.415.40636
• macOS 26.3
• MacBook Pro (Apple M3 Max, 16 CPU cores, 128 GB memory)

Observed

• Typing can become very slow, roughly ~1s for words to appear.
• Scrolling inside an existing conversation can become very slow.
• Scrolling the thread list can become very slow.
• UI updates such as archive notifications can arrive with noticeable delay.
• Switching into some older conversations can take a very long time.
• The app can also exit unexpectedly.
• This appears Codex-specific on my machine: the slowdown can show up immediately inside Codex while the rest of the machine feels normal.

Verified A/B Repro

I tested this by comparing copied local profiles rather than only changing one profile in place.

Verified results:

• A fresh test profile was smooth.
• A copied profile that kept most of my original conversations but excluded the largest bloated ones was also smooth.
• A copied profile that restored only a few very large historical conversations became slow again, even without the rest of my normal active conversations.

What this establishes:

• The strongest correlation I can verify so far is the presence of a few giant local histories, not raw total conversation count by itself.
• This is broader than thread-switch latency. The same profiles also show typing lag, scrolling lag, delayed UI updates, and occasional app exits.

What I have not isolated yet:

• Whether simply archiving those same conversations inside the main profile would be enough to fix it.
• Whether recurring thread heartbeat automations targeting those same heavy conversations are contributing independently, or mainly amplifying an existing history-size problem.

Hypotheses

These are hypotheses, not claims of proof:

• The desktop app may be over-hydrating, resuming, or indexing a few massive local conversation histories.
• Stale stored workspace roots may be adding background churn during startup and/or resume.
• Recurring thread heartbeat automations attached to already-large conversations may amplify the problem by repeatedly waking expensive local histories.

My current read is that the issue is likely caused by some combination of:

1. oversized local history payloads
2. resume/read/index churn on those histories
3. stale workspace-root scanning
4. possibly heartbeat-driven repeated wakeups on the same heavy conversations

Diagnostics

Largest local histories on this machine, anonymized:

Additional local signals:

• local state currently has 1767 total conversations, 777 unarchived
• 40 stored workspace roots no longer exist
• the desktop backend had 8 old session histories open totaling about 1.66 GiB
• desktop logs showed thread resume/read/unsubscribe operations taking tens of seconds, with worst cases around 82s
• startup also repeatedly scanned stale worktree/origin roots with very large expanded counts

Related issues

This may overlap with:

• #11011
• #18264
• #18333