Thanks a lot for the pointers!
The overall server status is pretty good. CPU usage is well under 10%.
Buffer cache hit ratio is almost 100%.
There is no developer to ask for code in my case. It is a third party application from vendor backed by SQL Server database. I think I will ask the user to run the app tasks that are taking long time and at the same time I do the profiler at the SQL side to capture what's going on. Then go from there.