Transfer rates increase during Computer use?

I'm not really sure where to place this, but I was just wondering if someone could explain something to me/why this happens.

I'm using SilverStack to transfer footage from a USB3 CFast Card reader to a Lacie Rugged 5400rpm 1TB Drive connected over F800. I have the reader connected to a Akitio Thunderbolt Hub which then goes to 13" 2011 MacbookPro.

When I'm allowing my computer to just sit and copy files, Silverstack reads that I am transferring to the drive at a rate of around 80-85 MB/s.

However, if I simply open up a program, like Chrome, and start surfing the web while the transfer happens, the transfer speed jumps to around 370 MB/s.

This is a phenomenon I've noticed a lot in the past,not just when using SilverStack, but regular file transfer as well, that when I just use simple programs on my computer the transfer speed increases from when I allow the computer to sit and do the transfer alone.

Why would using my computer help the transfer speed increase? I'm just trying to learn a bit more about how the computer splits usage.

Thanks!