Speed on table select or insert statements depend on many factors:
- table size - number of rows in table
- column type(s) - VARCHAR, INT...
- number of users in the database, including admin functions like running stored procedures.
- number of queries/inserts/deletes/edits on the table in question when your select statement runs. (Any resource contention, table locks)
- the server the database runs on - RAM, storage, other apps or DBs
- database settings cache, buffers
All of these things can impact your queries. Your index helps increase speed in certain statements (the ones that can use the data in the index without needing to search the entire table.) The index in itself is not a bad thing.
Now, speed changes: under normal operating conditions you should not see appreciable changes in query speed. (normal = average work day, abnormal = day when you have a large data load or delete, when another DB is created on that box and is in production, the power goes out and you're running on battery.) That being said, depending on how many concurrent users you have a 1 second difference in a query may be normal.
Here's what I look for if I suspect DB performance issues:
1. Any recent changes to the database or the server?
2. Is the running query substantially different from normal? A query with 4 joins will not be as efficient as a query on 1 table with no joins.
3. Any environmental issues lately? Such as power, unexpected reboots.
4. Review the DB logs and look for errors, locks and the like.
5. Review the server logs and look for errors.
6. Any network issues lately? Check with the network group on their logs.
7. Make a test query that is your baseline. You run this at the beginning of the database life, note the time to run and the number of rows returned. Then when you suspect things are not performing well, run your test query again. Is there a substantial change from your baseline? Additional rows and some increase in time are to be expected. Very different data, table lock on the query or a 10x slowing from your last test run would be unusual. If so, investigate why.
Try some or all of these steps and see if you discover any problems.