Next.js 16 increases TBT (Total Blocking Time)

[thernstig]

Link to the code that reproduces this issue

https://github.com/vercel/next-learn/tree/main/dashboard/starter-example

To Reproduce

I have a hard time saying how to reproduce this as I have a private website I cannot share, and reproducing this with a repro would be cumbersome. Hence the link to the Next.js tutorial. The page is fairly simple all-in-all with no custom client side JavaScript ('use client).

Current vs. Expected behavior

https://pagespeed.web.dev/ (Desktop) shows an increase in TBT after updating from Next.js 15 to Next.js and using cacheComponents: true where the page tested is using full static rendering during build time. Total TBT is 3,1 second on a fairly simple page.

This should be the culprit. This seems abnormally high for most of these

### ⚠️ Minimize main-thread work — 3.1s

Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this.

| Category                       | Time Used |
|--------------------------------|-----------|
| Script Evaluation              | 2,176 ms  |
| Other                          | 344 ms    |
| Garbage Collection             | 291 ms    |
| Style & Layout                 | 197 ms    |
| Script Parsing & Compilation   | 74 ms     |
| Rendering                      | 42 ms     |
| Parse HTML & CSS               | 13 ms     |

Provide environment information

This is from my local dev, although the results are seen on a live website. So Platform etc. is not relevant.

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP Tue Nov 5 00:21:55 UTC 2024
  Available memory (MB): 31662
  Available CPU cores: 32
Binaries:
  Node: 24.11.0
  npm: 11.6.1
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 16.0.3 // Latest available version is detected (16.0.3).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Performance

Which stage(s) are affected? (Select all that apply)

Vercel (Deployed)

Additional context

No response