  Th\^eo1 is a frequency stability statistic which is similar to the Allan
variance but can provide stability estimates at longer averaging factors and
with higher confidence. However, the calculation of Th\^eo1 is significantly
slower than the Allan variance, particularly for large data sets, due to a
worse computational complexity. A faster algorithm for calculating the
`all-$\tau$' version of Th\^eo1 is developed by identifying certain repeated
sums and removing them with a recurrence relation. The new algorithm has a
reduced computational complexity, equal to that of the Allan variance.
Computation time is reduced by orders of magnitude for many datasets. The new,
faster algorithm does introduce an error due to accumulated floating point
errors in very large datasets. The error can be compensated for by increasing
the numerical precision used at critical steps. The new algorithm can also be
used to increase the speed of Th\^eoBr and Th\^eoH which are more sophisticated
statistics derived from Th\^eo1.
