  Starting from a geometric perspective, we derive a quantum speed limit for
arbitrary open quantum evolution, which could be Markovian or non-Markovian,
providing a fundamental bound on the time taken for the most general quantum
dynamics. Our methods rely on measuring angles and distances between (mixed)
states represented as generalized Bloch vectors. We study the properties of our
bound and present its form for closed and open evolution, with the latter in
both Lindblad form and in terms of a memory kernel. Our speed limit is provably
robust under composition and mixing, features that largely improve the
effectiveness of quantum speed limits for open evolution of mixed states. We
also demonstrate that our bound is easier to compute and measure than other
quantum speed limits for open evolution, and that it is tighter than the
previous bounds for almost all open processes. Finally, we discuss the
usefulness of quantum speed limits and their impact in current research.
