Ajax select out of sync problem

There are two select when changing the option in the first select to request data from the server to load into SELECT2
This function is implemented but one bug is that the return data is too slow when you select the first select and then click SELECT2
Unable to synchronize usually wait a 1-2 seconds ask this question how to solve the novice one to ask God to help

Reply to discussion (solution)

The time taken to test the backend for the data

Need a back end every time? You can't put all the Select1->select2??? Into JSON? Like in the front?

If necessary? I beg the back end, can???? First select Change, clear the option of Select2, then use Ajax.

No, wait 1-2 seconds, it's slow.

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