audit existing nopCommerce application for performance

Probably, this is too generic question, but still I will try to ask.
I am very new to nopCommerce.

I have client who uses nopCommerce and his problem is performance.
I have to find out what makes their application so slow and provide suggestions for improvement..

As I am very new to nopCommerce, are there some advices that can be given on how to start?
What would you do first to find the problems? Which tools would you use?

If you haven't done so already verify that you are not running your production site in DEBUG mode.  It's an easy check that can drastically effect performance of a MVC application.

nopCommerce has a built-in profiler for .NET (miniprofiler.com) that can provide a little insight but you must enable it first

--Go to this URL for your site: http://<<your site>>/Admin/Setting/AllSettings
--Search the "settings name" column for "miniprofiler"
--Change the value to "TRUE"
--The mini-profiler will now appear in the upper-left of your site