If you have the Ignore local traffic within in the LAN option ticked, that's the expected behaviour. The network driver ignores any usage initiated by NetWorx itself. It can make the total figures less accurate, but unfortunately it's the decision of the driver's developers (we use a third-party driver). If this is your case, just wait a few minutes for the test to finish.
If you are using version 6.1 or below, there was a condition that could result in a Speed Test taking hours to finish. This has been fixed in the newer versions. If this is your case, please update to the latest version.