CSOM get user slow

Hello,

I've been executing the following code get a user to update a list-item of the type "User" by CSOM.

This has worked fine for months, but now suddenly, the ExecuteQuery-line takes more than 15 seconds...! I'm not getting any error but the performance is not acceptable!

Updating the item itself works fine...

What could be the cause of this performance issue ???

var newUser = clientContext.Web.EnsureUser(userName);
clientContext.Load(newUser);
clientContext.ExecuteQuery();
userValue.LookupId = newUser.Id;