I've been troubleshooting a SQL Server database that has recently started experiencing noticeable performance issues. Some queries that normally run quickly are now taking significantly longer, and I'm trying to determine the actual cause rather than simply adding indexes and hoping for an improvement. Slope

When you encounter this kind of situation, what do you normally check first? Do you start with the execution plans and query statistics, or do you investigate CPU, memory, disk I/O, blocking, and wait statistics before looking at individual queries?

I'm particularly interested in understanding a practical troubleshooting workflow for SQL Server performance problems. For example, which DMVs or built-in tools do you find most useful for identifying whether the bottleneck is caused by a poorly optimized query, missing or inefficient indexes, blocking, resource pressure, or something at the server level?

I'd appreciate hearing how experienced SQL Server administrators approach this kind of problem and which checks they consider essential before making any changes to the database.