How to make the synchronous requests better

I have an application in the following structure which will have a list of items.

Item_1  +   -
Item_2  +   -
  .     +   -
  .     +   -
  .     +   -
Item_N  +   -

The user can add, delete, modify, reorder the items in whatover order the user wants.
On each action(add, delete, modify, reorder), an update request will be sent to server side which will update all the items details.
The problem is the action is now getting triggered in asynchronous ajax requests which causing issue in the server side when faster actions made(which needs to be done in a sequential order).

One solution I got is changing the request from asynchronous to synchronous. But the problem is it’s blocking the UI during synchronous ajax call which looks like a lagging in the UI Perspective.

Is there anyway to avoid such lag while making the synchronous request or anyother solution for doing this uodate in the sequential order with asynchronous request?