How can we make the performance faster when creating a PO?

Our client needs 6 minutes to create one PR, with details below:

• in page Process Order> klik button Create > need 10 seconds until Edit Document PO openned
• when edit Base Price, each line needs 3 seconds, so for PO with only 6 lines needs 18 seconds for update the prices
• when update Tax Classification per each line needs 3-7 seconds. so for PO with only 6 lines needs arround 30 seconds for update tax classification
• when klik Save needs 4 seconds

How can we make the performance faster when creating a PO?