Textbox response is slow in iPhone safari browser

I have a web application running under windows server 2012 R2 & IIS 8.0. In iPhone 6, when I select a text box, it freezes for about 10 seconds before the keypad loads and I can type. In iPhone 5S, when I select a text box, the keypad loads immediately. But it textbox freezes for about 10 seconds before I can type. Other controls like dropdown list works fine with out delay. There are no issues in other phones.