Performance Issues with Federated Authentication and Virtual Users in Sitecore

Sitecore's Federated Authentication functionality works great and only needs a small amount of code and configuration to get up and running. During a recent upgrade to 9.0 Update 2 we ran into an interesting issue where Sitecore was reading and writing huge amounts of data to the ClientData table.

It turns out that Sitecore never expected both Federated Authentication and Virtual Users to be used at the same time. Typically Federated Authentication would handle everything for you but in our case we had a single sign on (SSO) implementation that wasn't quite compatible that drove use to create virtual users as part of the SSO process.

Sitecore Support created a patch for us that in their words:

Fixes excessive ClientData rewrites caused by LoginVirtualUser method when Federated Authentication is enabled.

The site is now running great with the patch in place.