This theme powers thousands of sites without complaints such as that, hopefully you can pass on a referenced list of issues that causes this slow down.
The theme is certainly responsive, you are using a 3rd party plugin which is causing this issue and we are trying to help you fix this. We cannot guarantee all 3rd party plugins will work, you can see the default theme demo is responsive.
The login details are no longer working, can you check those and update so we can view the checkout for you again?