In general you should't be storing documents of big size in SharePoint. A better approach would be to store it on a file share or FTP and simply link to the file from SharePoint.
You technically cannot store a file larger than 2 GB (2047 MB) to SharePoint. This is the maximum size. If you want to allow files up to 2 GB you'll need to increase the connection time-out in IIS and increase the maximum upload size for the web application.
1.SharePoint limit's the upload to 2GB due to IIS's worker process w3wp.exe, to upload a file you need to use all the IIS available memory to upload the full stream. Each w3wp.exe worker process runs well with 2-4GB of memory, this is not a boundary just a good idea (on x64), therefore this makes sense to me that the SP2010 team have limited any file upload to 2GB. If you start to download the file , your site would be very slow because all the IIS process is used for downlaoding the large documents.
2.Also be aware that increasing you upload file size to 2 GB has performance ramifications so it a user uploads a file and there is no memory available no new requests can be handled until the memory is available again.
I am sure there will be performance sure with huge GB of data.