Mac lag terribly when restarted from chosen Startup Disk My Mac has Filevault from quite some time ago.  Just now, I restarted it and went into recovery mode using cmd+R.

From there, I select Startup Disk from the menu, unlocked, and clicked restart.

After that, it restarted to the login screen and the computer started lagging really terribly at the startup screen. Even when I make furthur restarts by clicking on the Restart button at the login screen, the lag does not go away.

Does anyone know what is causing the lag?

(The lag went away when I clicked shutdown and turned on the computer.)
did you check for disk errors in Disk Utility?  Verify the disk, and repair if it finds anything.

If this is a sudden change, you could have either:  


a full disk
a failing drive
corruption