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 IOqueries in SQL Server database with following query.
select SUBSTRING(st.text,(qs.statement_start_offset/2)+1, ((case statement_end_offset when -1 then DATALENGTH(st.text) else qs.statement_end_offset end - qs.statement_start_offset)/2) +1) as statement_text, qs.total_logical_reads, qs.total_physical_reads, qs.execution_count from sys.dm_exec_query_stats as qs cross apply sys.dm_exec_sql_text(qs.sql_handle) as st order by qs.total_logical_reads desc, qs.execution_count desc
Query result will be like following screenshot