Encrypting Time Machine backup?

Do you encrypt your time machine backup? I have never done so with my older iMacs but I chose to do it with this new setup. When I hooked up my new external HD for the first time it asked me if I want to encrypt and I chose to. What are the pros/cons to doing this? I know it will protect my data. Does it slow my Time Machine backup down or does it not affect it at all? I am working with a fully loaded 27" iMac ( i7, 3TB Fusion, 32gb Ram, 2gb Video) and am doing the Time Machine backup via USB 3. I am about 20 minutes into it and have backed up about 23 gigs so far. I will stop and start over though if you all think it will slow things down.

Thanks for your time!

FYI: Its been about 60 minutes since I started the Time Machine backup and my progress is 265 GB of 595 GB saved so far with an estimate of "about an hour" left. That is saving the info via USB 3.0 with encryption.

UPDATE: So it finished saving everything to the external a while ago but its been sitting at this for a while now. I don't know if it is actually supposed to move from this spot or if there is some kind of bug happening. Its just stuck at 6%.

UPDATE: about 6.5 hours later and its still only at 47%. Does anyone have any knowledge on this. Is this the normal amount of time it takes to encrypt? Is it encrypting the entire 3TB drive or just the info I put on it? I am just trying to figure out if I should be concerned or not. I am tempted to just reformat and do a regular TM backup. Everything is saved on the external but its just going through this encryption phase now.

Thank you. Finally a response from someone! lol

Why does it take so long to encrypt? What is the process that it goes through that makes it so slow? Its encrypting the data I put on there and also the empty space?

Thanks for the info