Smith - if server is slow then we should go with below approach -
1) Check network latency
2) Check disk I/Os - can be checked with performance monitor toll in windows
3) Check CPU usage - can be checked with performance monitor toll in windows
4) Check memory usage - can be checked with performance monitor toll in windows
If it is related to any specific database -
we must at least check the below things
1) Blocking
2) Check processes using sys.sysprocesses
3) Check if any intensive query is running which can cause slowness for sometime until it finishes
4) Check if any database maintenance job is running
Let me know if you find something.