[BUG] Significant response slowdown since ~May 24 (v2.1.150) on Sonnet 4.6

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Starting around May 24 (coinciding with the upgrade from 2.1.145 → 2.1.150), response generation has become noticeably and consistently slower across all sessions. The slowness is throughout generation, not just time-to-first-token.

What Should Happen?

• Claude Code version: 2.1.152 (current)
• First affected version: 2.1.150 (installed May 24)
• Model: claude-sonnet-4-6 (explicitly pinned in ~/.claude/settings.json)
• OS: macOS 14.6 (Darwin 24.6.0)
• Auth: OAuth (Claude Code subscription, not API key)

Error Messages/Logs

Steps to Reproduce

No local configuration changes on my end. The slowdown correlates directly with the 2.1.150 update. Cannot determine if this is a client regression or a server-side change (compute reallocation away from Sonnet 4.6 toward newer models).

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.145

Claude Code Version

2.1.152 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_