Add note about compilation time to qc metrics

scanpy/preprocessing/_qc.py

@@ -60,6 +60,9 @@ def describe_obs(
     Calculates a number of qc metrics for observations in AnnData object. See
     section `Returns` for a description of those metrics.
 
+    Note that this method can take a while to compile on the first call. That
+    result is then cached to disk to be used later.
+
     Params
     ------
     {doc_adata_basic}
@@ -223,6 +226,9 @@ def calculate_qc_metrics(
     `Returns` for specifics. Largely based on `calculateQCMetrics` from scater
     [McCarthy17]_. Currently is most efficient on a sparse CSR or dense matrix.
 
+    Note that this method can take a while to compile on the first call. That
+    result is then cached to disk to be used later.
+
     Parameters
     ----------
     {doc_adata_basic}