Why Downloads calculate file size to download on fly after downloads starts? That slows down download speed. The file size exist on file record in database. When downloading the file it keep calculating it while downloading. Do I miss anything?

In my IPB3 it knew file size when download started already.

I have tagged our developers here, who are better placed to respond to your question. 

See my reply here.

Because we use ranges to send the contents of the file in parts, we need to also send the total file size of the file being downloaded along with the parts we are downloading. This needs to be completely accurate otherwise the download will fail.

Additionally, using ranges is not specific to the downloads application - as such, the size of a file may not be known when it is sent to the browser to be downloaded, so we get the total file size on the fly.

That being said, there should not be any significant impact on download speeds for this, since it is only done once at the start of the request. Any impact would likely be server related.