Customize Download Button

How can I improve or customize the download button? The download is extremely slow to initiate, the files are generally 40mb. So instead of users having to click the button multiple times and not getting a response, how can I improve on this delay? Any suggestion? Perhaps is it possible to make it work with the progress bar? Perhaps those countdowns Your download will begin shortly in 5…4…3…2…1 sec, how do I implement it?