PERF: Performance improvement on dataframe.update. 
the setitem under the hood is doing df copy per https://github.com/pandas-dev/pandas/issues/46267
therefore, the performance is compromised, per the suggestion, using loc for assignment instead 
see the detail example in the issue link for metrics.

- [X] closes #47392 (Replace xxxx with the Github issue number)

~Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature~
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

~Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~
~Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.~

@phofl  would you mind taking look at this PR @warm200 Thanks for the PR. https://github.com/pandas-dev/pandas/pull/47327 has been merged which includes the change in this PR so closing.