Note sub-optimal performance of cohort_statistic

sgkit/stats/popgen.py

@@ -1015,6 +1015,9 @@ def observed_heterozygosity(
     variables.validate(ds, {call_heterozygosity: variables.call_heterozygosity_spec})
     hi = da.asarray(ds[call_heterozygosity])
     sc = da.asarray(ds[sample_cohort])
+    # NOTE: Performance of cohort_statistic is substantially slower than a numba
+    # JIT function which handles cohorts directly (i.e. avoids slicing the
+    # `hi` array by cohort).
     ho = cohort_statistic(
         values=hi,
         statistic=np.nanmean,