Taking too much time to load select multiple in angular

Question

I am creating a multiple selection select box to select multiple users, these are the lines to create the multiple selection

<select formControlName="userIds" id="userIds" multiple>
        <option *ngFor="let opt of data" [value]="opt.id" class = "form-control">{{opt.userId}}</option>
      </select>

But it's taking too much time to load the contents inside the select box. If we are removing the "multiple" option from the select box it will load fastly. How can we fix this issue?