Here it is)
There were 2 problems:
1) BUFFERSIZE problem described above
2) the whole file was read at once (significant lag and high memory usage for large files)

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** [bugs:#1287] CURLOPT_BUFFERSIZE is ignored when downloading file:// url**
**Status:** open-confirmed
**Created:** Wed Oct 02, 2013 08:41 AM UTC by Maxim
**Last Updated:** Wed Oct 02, 2013 01:22 PM UTC
**Owner:** Daniel Stenberg
When downloading a file, write buffer is always 16383 bytes, CURLOPT_BUFFERSIZE is ignored.
The test file is attached.
I'm using the latest version from git (as of 1 Oct 2013, commit 3d43a487813a94dc8521bd8d970725efbd922bf7)
For the given test file an input file is generated as follows:
> $ dd if=/dev/urandom of=/tmp/curlin.tmp bs=1 count=50000
and for that input file I receive the following output:
> handles OK
    write_callback: total 16383
    write_callback: total 16383
    write_callback: total 16383
    write_callback: total 851
    upload result: count 0, No error
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