After borrowing a Concurrent Use license from ArcGIS License Manager on a different subnet, ArcMap freezes for some time when performing a variety of operations, such as geoprocessing. However, checking out the license does not trigger the issue.
The issue only occurs when using a borrowed license from a remote ArcGIS License Manager in another subnet. Using a borrowed license from a local ArcGIS License Manager does not trigger the issue.
To prevent ArcMap from freezing while using a borrowed Concurrent Use license, the service.txt file on the license server must be modified to contain the Fully Qualified Domain Name (FQDN) of the license server.
C:\Program Files (x86)\ArcGIS\License10.x\bin
C:\Program Files (x86)\ArcGIS\LicenseManager\bin
SERVER this_host ANY 27000 VENDOR ARCGIS USE_SERVER