This is caused by faster assistants slowing down to the speed of the average - so as not to embarrass their slower colleagues - while the "slow ones, stay slow", which has an overall effect of dragging down the speed.

BBC: Good queue guide

Applying any of these effects simply involves selecting a clip, selecting the effect, previewing it, and then dragging-and-dropping the result onto the clip.

CNN: iMac DV makes home video editing simple