How to measure a perfomance of SQL Server database? I have the task to improve the performance of a SQL Server 2012 database (one of 4 in an instance) by 60% confirmed by corresponding statistics. 

So, I need to measure "performance" of the RDBMS database before performance tuning and optimization and after.

Which metrics are better suited for this? 

Trying to answer the obvious questions ahead ...

I/O (hardware) bottlenecks are absent since the SQL Server runs on a virtual rack having plenty of physical resources under it.  

The database is used by approx. 60 users (mostly 8 hours a day) with widely varying load (per sec).

This is a company management task, so the results of this task should be easy to grasp. 

UPDATE:
the corresponding my question was answered in StackOverflow.com by @Martin von Wittich and, then, deleted by moderator/community:  


  Run different kinds of queries (INSERT, UPDATE, DELETE) in a loop
  (e.g. 1000 times), then divide the runtime by the amount of loop
  iterations. Now you now how long a single query of that type takes.
  
  Then do whatever you want to do to improve the performance, and
  compare the results.


Quite good for me. Also I do not see why it was deleted in SO with the reason: "This question does not appear to be about programming" and closed in this site with: "There are either too many possible answers, or good answers would be too long for this format."
I/O (hardware) bottlenecks are absent since the SQL Server runs on a virtual rack having 
  plenty of physical resources under it.


Let's get this straight:


Most virtualization is pathetic - not even low, pathetic - on the IO side.
"having plenty" means nothing unless you qantify there is no IO problem.


At the end, come up with the metcis you need to measure, and find the bottlenexks. But starting with "virtual rack has no IO issues" will give you smiles from most people using virtualization.