When I download a lot of data (e.g. 3GB) in one go, using a program such as Transmission or Wget, progressively during the downloading, and after it has has finished, the computer always seems slightly sluggish, as though it's been using swap. However, the result of
free always shows 0 bytes of swap used, both during the downloading and after.
I typically notice that the program used to download is slower to close, and subsequent programs are slower to open, but only the first time they're opened, as though the data is being transfered from swap to RAM.
My swap and
free aren't faulty, since on other occasions
free reports some swap usage as expected. My computer is never suspended nor hibernated; I don't use a screen saver; my computer is switched off at the end of every day.
My computer has 4 GB of RAM, and a fast processor, which never goes above ~20% usage when/after I'm downloading.
I'm using Linux.
What could be the cause of this behaviour?