The reason why it's so fast in Pendulum, is that adding 100 items runs between two request animation frames. Ids of items are gathered in signals as lists and the instant function just apply the action to all elements of the list in the following requestAnimationFrame. So it takes only one logical instant which is basically here the execution of a list iterator.
Notice that Om, Mercury, and Elm consistently do really well compared to the other entries.
All three of these projects are based on the Virtual DOM approach and make heavy use of immutability to get these speed gains.
Read more about this here.
And here is the code used for these tests.