perf: optimizing @let

[cipchk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[cipchk]

Preview is ready!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.84%. Comparing base (7a40351) to head (d9e34cf).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1849   +/-   ##
=======================================
  Coverage   94.84%   94.84%           
=======================================
  Files         315      315           
  Lines        8930     8930           
  Branches     1802     1802           
=======================================
  Hits         8470     8470           
  Misses        382      382           
  Partials       78       78           

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