SQL Server built-In monitoring components

First, It should have been determined on where/what to monitor for the current situation. right ? Yes!

And, select an appropriate tools to proceed with. It can be either from Windows Itself Or SQL Server specific

Windows Monitoring Tools:

• System Monitor
• Performance Monitor
• Task Manager
• TypePerf

SQL Server Tools:

• Activity Monitor
• Database Console Commands (DBCC)
• Distributed Replay Utility
• Dynamic Management Views and Functions
• Extended Events
• Functions
• SQL Server Management Studio Graphical Showplan
• SQL Server Profiler
• SQL Trace
• System Stored procedures
• Trace flags

Note: 

• SQL Trace and SQL Server Profiler are deprecated. Avoid using this feature in new development work, and plan to modify applications that currently use this feature.
• Use Extended Events instead

Source: https://docs.microsoft.com/en-us/

Is always good idea to use Temporary Table ?

No. Its NOT

As far as I know, Sometimes using Temp table improves performance and Sometimes Its not!

Creating a Temp table to hold intermediate results forces SQL Server to interrupt the data pipeline through a query to persist the results to disk.

Sometimes just doing one query rather than pre-aggregating or pre-sorting can be way more efficient and lead to far lower run time and tempdb usage!

So, "Always not good using Temp table to have intermediate aggregations/processing before resulting it"

It has to be compared with various methods with/without Temp table/CTE/table variable before implementing it in production environment!

So, Its not the case always do something Or always not to do something.

Importantly, make sure to pull only the data that's really required and create Non-clustered index after the table populated when using Temp tables to gain proper statistics created on that with actual loaded data!

Also, Creating more Non-clustered Index that aren't actually being used also causes the slowness on loading data and retrieving as well!

So, Non-clustered index to be created only based on the query plan actually using. 

There is no point of pulling Millions of records which is even not needed at all. Right ? But, Its happening most of times which can lead to enormous Tempdb usage!!!