PERF: Avoid deep copies when casting dtypes in merge (#62902) #50207

Job	Run time
Test Conda Forge Recipe	2m 17s
Install Extras - plot	4m 8s
Install Extras - xml	3m 48s
Install Extras - output-formatting	3m 47s
Install Extras - all	4m 39s
Install Extras - aws	3m 58s
Install Extras - compression	3m 47s
Install Extras - postgresql	3m 53s
Install Extras - hdf5	3m 56s
Install Extras - fss	3m 55s
Install Extras - html	3m 48s
Install Extras - clipboard	4m 3s
Install Extras - test	3m 46s
Install Extras - spss	3m 41s
Install Extras - computation	3m 56s
Install Extras - pyarrow	3m 45s
Install Extras - feather	3m 43s
Install Extras - mysql	3m 45s
Install Extras - parquet	3m 41s
Install Extras - sql-other	3m 47s
Install Extras - performance	3m 55s
Install Extras - excel	4m 8s
Install Extras - gcp	4m 0s
	1h 28m 6s

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