sagropa, it depends on
1) loading of the server you are in
2) do not download/upload while packing
3) there are some abusers packing/unpacking multiple files
even if admin warn them NOT to pack/unpack multiple files at the same time
4) try to zero compression while making rar file, cause even if you choose
the best complression you will earn %5 smaller size.
this is I choose zero compression. and do nothing while compression