explain why Tor is slow

Originally created by Tails on #5641 (Redmine)

The lack of documentation about the performance hit (especially w.r.t delay) imposed by Tor usage was pointed out as a big source of TBB user confusion in a PETS paper: http://petsymposium.org/2012/papers/hotpets12-1-usability.pdf

How can we make this clearer for Tails end-users?

Feature Branch: doc/explain_why_Tor_is_slow