etseidl commented on PR #9831:
URL: https://github.com/apache/arrow-rs/pull/9831#issuecomment-4383977021

   So the regression is def there. Around 5% on average for the 
list_primitive's. I ran the list_primitive/default through samply, and it looks 
like a good bit of the slowdown is in `LevelInfoBuilder::write_leaf`. I'm 
seeing `extend` in main at 6100 samples, being replaced by `extend_from_iter` 
3480, extend 3070, append_rep_level_run 1890 (total ~8400). Looking at the 
code, the only real difference is passing through all the `match`s to pick the 
right enum variant.
   
   Would it be possible to have different versions of `write_leaf` for each 
variant?
   
   Honestly I'm far more tolerant of write regressions than read. And 5% just 
for lists seems a fair trade.


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