Encrypting contents of Macbook Air
Yes, you can activate or deactivate FileVault later in the settings. Of course there will be a performance drop when running FileVault, but only so little, that you would have a very hard time measuring the slow down let alone having it affect your system in any meaningful way.

The system might take 10 seconds longer to boot at most and have a pause where you have to enter your password before the rest of the normal start up happens.