Your computer has three drives in a RAID $5$ configuration. One of the drives fails so you replace it right away. Immediately upon replacing the drive you notice that the overall performance seems to have been degraded. You research the issue and conclude that this is the expected behavior. How can you rationalize this behavior?

You can't rationalize this behavior. It is a false statement. Performance should not be degraded.

It takes time to rebuild the data on the new drive. During this time performance is degraded.

When a RAID $5$ drive fails, the data is temporarily written to pagefile.sys which is slower than RAID $5.$

The new drive was not detected so the system continues to use RAID $5,$ but in a degraded mode.