  We use a specific geometric method to determine speed limits to the
implementation of quantum gates in controlled quantum systems that have a
specific class of constrained control functions. We achieve this by applying a
recent theorem of Shen, which provides a connection between time optimal
navigation on Riemannian manifolds and the geodesics of a certain Finsler
metric of Randers type. We use the lengths of these geodesics to derive the
optimal implementation times (under the assumption of constant control fields)
for an arbitrary quantum operation (on a finite dimensional Hilbert space), and
explicitly calculate the result for the case of a controlled single spin system
in a magnetic field, and a swap gate in a Heisenberg spin chain.
