Combobox take long time for display list

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CSS

ExtJS

on my page, there is list of nearby than 6k. the code in js
Ext.define('DCT.ComboField', {
extend: 'Ext.form.ComboBox',

shadow: false,
typeAhead: true,
forceSelection: true,
queryMode: 'local',
valueField: 'code',
displayField: 'display',
valueNotFoundText: '',
inputXType: 'dctcombofield',
dctControl: true,
keyDownValue: "",
anyMatch: true,
xtype: 'dctcombofield',
defaultListConfig: {
loadingHeight: 70,
minWidth: 30,
maxHeight: 300,
shadow: 'sides'
},
it takes long time to display list. more than 2 sec.

What I have tried:

i want to add minLength . but need ask the find position.