Why does JS slow down on the server?

Question

JavaScript

Trying to make a choice of seats in the theater.
1) I use dragdealer.js for dragging (although the same problem with jquery-ui draggable)
Since a large number of seats (divs) start to slow down when dragging on the server, BUT if you download the page to yourself or run it on local there are no lags.
https://codepen.io/kepo4ka/pen/zRZYzx
Even worse when installed on a wordpress page.
Why such a situation?
(even considering the terrible code, it works fine on the computer)

Answer 1

dyg. 900 elements per page... which browser will pull it off? no. iPad dies in general.
Yuzai Alex <map>-1917 is talking