  Laser speckle can provide a powerful tool that may be used for metrology, for
example measurements of the incident laser wavelength with a resolution beyond
that which may be achieved in a commercial device. However, to realise highest
resolution requires advanced multi-variate analysis techniques, which limit the
acquisition rate of such a wavemeter. Here we show an arithmetically simple
method to measure wavelength changes with dynamic speckle, based on a
Poincar\`e descriptor of the speckle pattern. We demonstrate the measurement of
wavelength changes at femtometer-level with a measurement time reduced by two
orders of magnitude compared to the previous state-of-the-art, which offers
promise for applications such as speckle-based laser wavelength stabilisation.
