Drastic perf decrease in {#each} block performance from v5.43.8+

[MathiasWP]

Describe the bug

After upgrading to v.5.43.8 our application started freezing in places where we use a VirtualList component. It seems like something was introduced in #17150 which causes this performance regression.

Reproduction

Go to this REPL and scroll up and down the page enough times, and the whole website just freezes: https://svelte.dev/playground/b6b5a280e01b4996bb1b3499f721fa46?version=5.43.10

Video:

Screen.Recording.2025-11-18.at.12.08.31.mov

Logs

System Info

System:
    OS: macOS 15.6.1
    CPU: (8) arm64 Apple M1 Pro
    Memory: 94.16 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - /Users/mathiaspicker/.nvm/versions/node/v22.14.0/bin/node
    npm: 10.9.2 - /Users/mathiaspicker/.nvm/versions/node/v22.14.0/bin/npm
    pnpm: 10.15.1 - /opt/homebrew/bin/pnpm
    bun: 1.0.0 - /Users/mathiaspicker/.bun/bin/bun
  Browsers:
    Brave Browser: 142.1.84.139
    Chrome: 142.0.7444.176
    Edge: 142.0.3595.80
    Firefox: 142.0.1
    Safari: 18.6

Severity

blocking an upgrade