fix(opencode): Throttle storage writes during streaming to reduce I/O overhead

[taetaetae]

Problem

During streaming responses, OpenCode writes to storage (JSON files) on every reasoning-delta event. With fast streaming APIs, this results in hundreds of file writes per second, causing:

1. High I/O overhead from frequent file system operations
2. Increased latency between receiving content and displaying it
3. Slower perceived response time compared to native CLI tools (e.g., kiro-cli)

Fixes #11329

Analysis

I compared OpenCode with kiro-cli using the same Kiro API:

Aspect	kiro-cli	OpenCode
API endpoint	Same (q.us-east-1.amazonaws.com)	Same
TTFB	2-10s	2-10s
Delta handling	Memory only	File write per delta
Perceived speed	Fast	Slower

The bottleneck is in processor.ts:

case "reasoning-delta":
  if (part.text) await Session.updatePart({ part, delta: value.text })
  // This writes to file system on EVERY delta!

Session.updatePart calls Storage.write which does:

await Bun.write(target, JSON.stringify(content, null, 2))

Solution

Implement throttled storage writes for reasoning-delta events:

• Accumulate content in memory (already happening via part.text +=)
• Only flush to storage every 50ms (STREAMING_UPDATE_THROTTLE_MS)
• Always flush on reasoning-end to ensure data persistence
• No change to final content or persistence guarantees

Expected Impact

• ~20x fewer file writes during streaming (assuming 1 delta/2.5ms average)
• Reduced I/O overhead and CPU usage
• Faster perceived response time
• No change to final message content

Changes

• Added STREAMING_UPDATE_THROTTLE_MS constant (50ms)
• Added throttle state tracking (lastUpdateTime, pendingReasoningUpdates)
• Modified reasoning-delta handler to throttle storage writes
• Ensured reasoning-end always flushes pending updates

[github-actions]

Hey! Your PR title perf(processor): Throttle storage writes during streaming to reduce I/O overhead doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[rekram1-node]

we are moving to sqlite