File download question

I made successfully a file download center which so far works awsome, unless the downloadable files are getting a bit big.
As soon as there are bigger files involved, one has to wait a considerable time until the file is ready for saving, which is ok and can be expected.

What maybe would be nice to have is a message poping up, similar to the form progress bar or maybe a little tutorial about how to make it.

The bigger issue I run into, was that while waiting for the file to be ready for download, the entire site got blocked until the file was ready.

I wonder if I missed a detail somewhere while setting up the DL center or if it maybe possible to set up a read only database instance on the same server with the dl infos to avoid the rest of the site being blocked ?