We run some high volume Drupal Commerce sites, taking upwards of 10,000 orders an hour and have done a lot of performance work to get these values. Commerce traditionally can't be cached and tuned the same way an unauthenticated content site can, and normal tricks like varnish and other static caching are of limited use.

An overview of performance problems with Drupal Commerce and Drupal 7 and ways to fix or mitigate them.

Caching with authenticated users
Caching with dynamic content
Scaling Orders
Database Locking
Order Locking
PHP 7 benefits specific to Commerce
Scaling Attributes and Options
I will also touch on changes coming with Commerce 2.x for Drupal 8 and how this will affect performance in Drupal, especially related to cache contexts and bigpipe.

After this talk, Commerce site admins should be able to increase there order throughput hopefully by a factor or 3x or more, depending what there existing site setup is.

Blog post example regarding locking in commerce: https://www.acromediainc.com/blog/drupal-commerce-performance-locking