If you got slowness complaint from customer, you need to monitor SQL Server Instance and database which sql is consuming a lots of resource.
SQL Server DBA should monitor database everytime and if there are many sqls which is running long execution time or consuming a lots of IO resource then it should be reported to the developer and developer and dba should examine these sqls.
You can find TOP 50 IO queries in SQL Server database with following query.
select q.[text], SUBSTRING(q.text, (highest_cpu_queries.statement_start_offset/2)+1, ((CASE highest_cpu_queries.statement_end_offset WHEN -1 THEN DATALENGTH(q.text) ELSE highest_cpu_queries.statement_end_offset END - highest_cpu_queries.statement_start_offset)/2) + 1) AS statement_text, highest_cpu_queries.total_worker_time, highest_cpu_queries.total_logical_reads, highest_cpu_queries.last_execution_time, highest_cpu_queries.execution_count, q.dbid, q.objectid, q.number, q.encrypted, highest_cpu_queries.plan_handle from (select top 50 qs.last_execution_time, qs.execution_count, qs.plan_handle, qs.total_worker_time, qs.statement_start_offset, qs.statement_end_offset, qs.total_logical_reads from sys.dm_exec_query_stats qs order by qs.total_worker_time desc) as highest_cpu_queries cross apply sys.dm_exec_sql_text(plan_handle) as q order by highest_cpu_queries.total_logical_reads desc;