Notes
Querying the database: In Apex, querying the database can be an expensive operation, especially if the query is performed repeatedly within a loop. Instead, it's best to retrieve all necessary data outside of the loop and then process it within the loop.
Performing DML operations: Similarly, performing DML operations, such as inserting, updating, or deleting records within a loop can be inefficient and can also cause governor limit issues. It's best to accumulate the changes within the loop and then perform the DML operations outside of the loop.
Calling web services: Making web service calls within a loop can also be a performance bottleneck, especially if the web service is slow or has a high latency. Instead, it's best to make the necessary web service calls outside of the loop and then process the results within the loop.