chore: minimization of variable-induced slowdown

[semorrison]

I minimized this from an example @kbuzzard and @sgouezel found in Mathlib's slowest file.

The basic example is:

-- ... First setup a maze of typeclasses from Mathlib ...

example : Nat := 0

variable [Algebra R S]

example : Nat := 0 -- now takes more than 15x as many heartbeats!

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 2bc41d8f3aced0afb6e8d98dbd81d7d7a325da50 --onto ff37e5d512efcd3981290270a2fc3ecb100bbd0c. (2024-05-23 01:17:41)