slightly better performance

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (74b32f6) 96.35% compared to head (2bb8c13) 96.45%. Report is 1 commits behind head on master.

Files	Patch %	Lines
src/fit.jl	96.59%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #257 +/- ## ========================================== + Coverage 96.35% 96.45% +0.09% ========================================== Files 8 8 Lines 659 648 -11 ========================================== - Hits 635 625 -10 + Misses 24 23 -1 ```

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