Form is responding slow when the fields are being selected

Loading it working fine but when I selected each items that when it slow to show it select.

You are right. As mentioned in the earlier post, this should be happening due to the enabled auto-fill feature on the form with more than 300 fields. When any form field is selected/changed, the data will be stored in the browser storage. You may try disabling this option and see the difference. If you need to store the form data to continue later, you may consider using session parameter method.

https://www.jotform.com/help/97-How-to-Save-Forms-to-Continue-Later

Hope this information helps!