SQL Server wait stats
The single biggest clue to the source of a performance problem will be from something referred to as the wait stats.
What are waitstats? In short wait stats are statistics gathered on what resources SQL Server is waiting on internally whilst executing queries.
Why are they useful? They can often provide very quick and objective evidence of a performance bottleneck by showing cumulative wait times that SQL Server spent on getting access to the CPU, memory or disk access as well as numerous other internal resources.