I'd start with slow running queries;

if you cannot change the app code, you might at least add indexes that will help.

here's a query for the top 20 slowest queries

SELECT TOP 20 object_schema_name(qt.object_id, qt.database_id) + N'.' + object_name(qt.object_id, qt.database_id) AS 'SPName', qt.query_text , qs.total_worker_time AS 'TotalWorkerTime',

qs.total_worker_time/qs.execution_count AS 'AvgWorkerTime',

qs.execution_count AS 'ExecutionCount',

ISNULL(qs.execution_count/DATEDIFF(Second, qs.creation_time, GetDate()), 0) AS 'CallsPerSecond',

ISNULL(qs.total_elapsed_time/qs.execution_count, 0) AS 'AvgElapsedTime',

qs.max_logical_reads, qs.max_logical_writes,

DATEDIFF(Minute, qs.creation_time, GetDate()) AS 'AgeInCache'

FROM sys.dm_exec_query_stats AS qs

cross apply msdb.MS_PerfDashboard.fn_QueryTextFromHandle(sql_handle, statement_start_offset, statement_end_offset) as qt

WHERE qt.database_id = db_id()

ORDER BY qs.total_worker_time DESC

here's queries taking longer than 15 seconds:

select

fn.*,

st.*

from sys.dm_exec_query_stats st

cross apply sys.dm_exec_sql_text(st.[sql_handle]) fn

where st.max_elapsed_time >= (1000 * 15) --15 seconds