... is probably this. Clean your keyboards! When was the last time you did it? I clean mine once in 2 months and the dirt accumulated is quite horrible. Partly because the mac keyboard is white & transparent so seeing the dirt really annoys me. The amount of gunk embedded inside the board: skin cells, grease, hair, eraser bits, dust, eyelashes and dead flies even. Don't forget we sometimes sneeze, cough and eat while typing (I avoid eating around the computer area now after spilling sugared drinks onto the board)!
Don't have time to take apart my keyboard this time so just used alcohol to wipe on top and in between the keys. I go through alcohol bottles rather quickly because the mouse is cleaned every few days and the keys every few weeks. Now it's running smooth and don't feel 'stuck' anymore so I can get back to work!