Firstly never get Demotivated due to the kind interviewer's perception of your answers , its possible he/she may even have no idea that something that you answered even exists .

I been asked about if I have seen the Racks and hardware of my Data center , being a DBA I should have but it was restricted to few people based on company policy . So I fumbled in that . Anyways what I will check for query performing poorly , not saying what you answer is incorrect .

I will first ask is any recent changes have been in the code , like any deployment or anything like that .

No query will start behaving super slow with out any big change , lot of people miss that and jump directly chasing the goose .

1. Actual Execution plans , which tells a lot about operator consuming more cost and if required Index is missing or not ( Read Book by Grant , I guess the only Awesome book on Execution plan) .

2. Logical reads , which causes slowness for sure . SET STATISTICS IO ON etc .

3. Isolation level , table hints .

4. Wild cards , Joins and clauses .

The list probably goes further on type of query , what type objects included . I am sure there are more items and its extensive. Hope it help to some extent .