My mid 2011 MacBook Air has slowed down significantly overnight?

Last night my MacBook Air was working fine.

When I went to use it this morning, it has slowed down significantly. It takes awhile for it to respond to trackpad movement or the keyboard. Opening finder is delayed now, opening any app is delayed now. Booting it back up takes extra long now, probably around a minute compared to thirty seconds before.

I've tried an SMC reset, repairing the disk and repairing disk permissions.
From what I can see, the hardware doesn't appear to be failing, but I could be wrong.

Can anyone help me out? What could possibly be going on with my MacBook? How did this happen overnight?

 

Nothing downloaded last night.
And I had the lid closed, too.
So I really don't know what happened.