Vue slot is updated after any data changed, even there is no dependency

[hzsrc]

When we use slot, the component in the slot is updated after any data changed, even there is no dependency. Here is a sample(see console):

https://codepen.io/hzsrc/pen/gbYxOry

When this is used in a big component with hundreds of sub components, it cause big performance problems, the app runs very slowly.

Why this happen, is it a bug? How to avoid this ?