Block updates, such as pistons, observers, and other redstone components, cause noticeable amounts of lag. Attached are some screenshots.
As you can see, when the piston isn't moving, I have around 60 frames. When I create a simple observer clock to update that piston, my frames are cut in half.
To recreate:
Create a new world
Create a redstone clock, or any mechanism that repeatedly updates a block.

Yes, I'm well aware this is a duplicate, but seeing as the original issue didn't have screenshots attached and wasn't very well documented, I thought I'd provide a bit more information. Hope it helps :/