how google lighthouse calculates javascript evaluation time and why it varies by a huge margin for same scripts on different environments

I'm working on improving the performance scores of a website. On running lighthouse in Reduce JavaScript execution time under diagnostics, I see high Script Evaluation time for few of my js scripts.

If I run lighthouse on different environment I get different evaluation time for the same scripts, this difference is very large for instance for one script it changes from 1700ms to 100ms,

I wanted to understand how lighthouse calculates the evaluation time, so I can work on reducing it.

Those js files are comparatively smaller than other files having lower evaluation time, which I'm unable to understand. If it matters, in profiling I can see that execution time of the same script is 45ms.

PS- I know that script execution and evaluation are different

Thanks any help would be much appreciated.

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