Speed up heat equation

[larsevj]

Issue
Resolves #my_issue

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.98%. Comparing base (7aa972d) to head (08396a9).
⚠️ Report is 13 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13486      +/-   ##
==========================================
+ Coverage   89.95%   89.98%   +0.02%     
==========================================
  Files         459      460       +1     
  Lines       32173    32320     +147     
==========================================
+ Hits        28942    29083     +141     
- Misses       3231     3237       +6     

Flag	Coverage Δ
cli-tests	37.22% <ø> (-0.12%)	⬇️
fuzz	44.07% <ø> (-0.08%)	⬇️
gui-tests	66.66% <ø> (-0.14%)	⬇️
performance-and-unit-tests	78.05% <ø> (+0.08%)	⬆️
test	45.62% <ø> (-0.16%)	⬇️

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see 12 files with indirect coverage changes