There's no extra risk associated with converting a 80% full disk. That should work fine without any problems.
After encryption you'll have about the same amount of disk space available. It is not going to result in any kind of noticeable drop in available disk space.
The Mac will run a tiny bit slower compared to before. I doubt that you will be able to notice it at all.
You can keep using the computer while it encrypts. It is impossible for us to predict how long the encryption process will take - but expect a considerably number of hours, perhaps a full day. It runs in the background allowing you to keep using the computer - and it supports you putting the laptop to sleep or even turning the power fully off. It wil resume where it left off when it is powered on again.