CPU is P8600 @ 2.4 GHz (average laptop)
All folders in the same partition.
The problem does not exist with smaller files, as you point out. As the download of the file takes X seconds, and the decode takes perhaps 0.2 X seconds, it would be better if the resources used by the decode would be less. The decode would perhaps take 0.5 X seconds (half the time of the download of a rar file), which is quick enough to keep up with the download. Perhaps then the download could keep up at full speed, overall speeding up the download.