Improve troubleshooting guidance for copy activity performance

Clarified the explanation for long duration in 'Transfer - Time to first byte' and provided additional context on potential causes and actions.

Author: whhender

articles/data-factory/copy-activity-performance-troubleshooting.md

@@ -58,7 +58,7 @@ When the copy activity performance doesn't meet your expectation, to troubleshoo
 
 - **"Pre-copy script" experienced long duration:** it means the precopy script running on sink database takes long to finish. Tune the specified precopy script logic to enhance the performance. If you need further help on improving the script, contact your database team.
 
-- **"Transfer - Time to first byte" experienced long working duration**: it means your source query takes long to return any data. Check and optimize the query or server. If you need further help, contact your data store team.
+- **"Transfer - Time to first byte" experienced long working duration**: your source query takes a long time to return any data. This could mean the query takes a long time to process on your source because the source is busy with other tasks, or the query is not optimal, or the data is stored in such a way that it takes a long time to retrieve. Consider if other queries are running on that source at the same time, or if there are any updates you could make to your query so it can retrieve data more quickly. If there is a team that manages your data source, contact them to modify your query or check source performance.
 
 - **"Transfer - Listing source" experienced long working duration**: it means enumerating source files or source database data partitions is slow.
   - When copying data from file-based source, if you use **wildcard filter** on folder path or file name (`wildcardFolderPath` or `wildcardFileName`), or use **file last modified time filter** (`modifiedDatetimeStart` or`modifiedDatetimeEnd`), note such filter would result in copy activity listing all the files under the specified folder to client side then apply the filter. Such file enumeration could become the bottleneck especially when only small set of files met the filter rule.