Why would reusing a DataContext have a negative performance impact?

Question

After a fair amount of research and some errors, I modified my code so that it creates a new DataContext each time the database is queried or data is inserted. And the data

Answer 1

Not exactly on point here, but have you considered some sort of application-level cache to look up the customer id, department id, and category? It's not clear from your post how many of these entities exist in your system, or what is involved in querying to obtain them.

However, as an example, if you have one million categories in your system, and you need to look up their Id by category name, keeping an name/Id dictionary in memory for lookup at all times will save you a trip to the database for transaction you process. This could massively improve performance (this assumes a few things, like new caregories aren't being added regularly). As a general rule, round trips to the database are expensive relative to in-memory operations.