I have a 50 gb data in my org for one object. My triggers will fire and upsert a record on it, does the volume of data show any impact in performing the UPSERT DML operation.
It will have approximately the same performance as it would to query the records by External ID and make the decision manually. In other words, there's no performance benefit from not using upsert. This is because External Id is indexed, so records can be identified very quickly.
You can put a system.debug() statement before and after the DML statement. Then you can test in the production database and in an empty sandbox. The log file will show you for sure if there's a difference. Be sure to test a few times and take an average.
