Matthew McGiffen pulls out the magnifying glass with a built-in light:
The encryption of your existing data occurs as a background process referred to as the encryption scan, but it will consume resources while it runs, so if you are implementing TDE against a system with large databases where performance is critical then you will want to either run it in a period of quiet (or down time), or you will want to monitor to check that encryption isn’t impacting your system too much. Experience suggests that it shouldn’t be a problem unless your server is already under strain.
There are a few things to look out for if you are monitoring during the encryption scan:
Click through for a list of items you might want to keep in mind, as well as some important tips about suspending or stopping the encryption process.