diff --git a/crates/polars-core/src/chunked_array/from_iterator_par.rs b/crates/polars-core/src/chunked_array/from_iterator_par.rs
index 12263053e368..eaf45d1d651f 100644
--- a/crates/polars-core/src/chunked_array/from_iterator_par.rs
+++ b/crates/polars-core/src/chunked_array/from_iterator_par.rs
@@ -82,21 +82,21 @@ where
 {
     fn from_par_iter<I: IntoParallelIterator<Item = Option<T::Native>>>(iter: I) -> Self {
         let chunks = collect_into_linked_list(iter, MutablePrimitiveArray::new);
-        Self::from_chunk_iter("", chunks)
+        Self::from_chunk_iter("", chunks).optional_rechunk()
     }
 }
 
 impl FromParallelIterator<bool> for BooleanChunked {
     fn from_par_iter<I: IntoParallelIterator<Item = bool>>(iter: I) -> Self {
         let chunks = collect_into_linked_list(iter, MutableBooleanArray::new);
-        Self::from_chunk_iter("", chunks)
+        Self::from_chunk_iter("", chunks).optional_rechunk()
     }
 }
 
 impl FromParallelIterator<Option<bool>> for BooleanChunked {
     fn from_par_iter<I: IntoParallelIterator<Item = Option<bool>>>(iter: I) -> Self {
         let chunks = collect_into_linked_list(iter, MutableBooleanArray::new);
-        Self::from_chunk_iter("", chunks)
+        Self::from_chunk_iter("", chunks).optional_rechunk()
     }
 }
 
@@ -106,7 +106,7 @@ where
 {
     fn from_par_iter<I: IntoParallelIterator<Item = Ptr>>(iter: I) -> Self {
         let chunks = collect_into_linked_list(iter, MutableBinaryViewArray::new);
-        Self::from_chunk_iter("", chunks)
+        Self::from_chunk_iter("", chunks).optional_rechunk()
     }
 }
 
@@ -116,7 +116,7 @@ where
 {
     fn from_par_iter<I: IntoParallelIterator<Item = Ptr>>(iter: I) -> Self {
         let chunks = collect_into_linked_list(iter, MutableBinaryViewArray::new);
-        Self::from_chunk_iter("", chunks)
+        Self::from_chunk_iter("", chunks).optional_rechunk()
     }
 }
 
@@ -126,7 +126,7 @@ where
 {
     fn from_par_iter<I: IntoParallelIterator<Item = Option<Ptr>>>(iter: I) -> Self {
         let chunks = collect_into_linked_list(iter, MutableBinaryViewArray::new);
-        Self::from_chunk_iter("", chunks)
+        Self::from_chunk_iter("", chunks).optional_rechunk()
     }
 }
 
@@ -136,7 +136,7 @@ where
 {
     fn from_par_iter<I: IntoParallelIterator<Item = Option<Ptr>>>(iter: I) -> Self {
         let chunks = collect_into_linked_list(iter, MutableBinaryViewArray::new);
-        Self::from_chunk_iter("", chunks)
+        Self::from_chunk_iter("", chunks).optional_rechunk()
     }
 }
 
diff --git a/crates/polars-core/src/chunked_array/mod.rs b/crates/polars-core/src/chunked_array/mod.rs
index 05d682fc6ed9..72d1f1e53813 100644
--- a/crates/polars-core/src/chunked_array/mod.rs
+++ b/crates/polars-core/src/chunked_array/mod.rs
@@ -144,6 +144,19 @@ pub struct ChunkedArray<T: PolarsDataType> {
 }
 
 impl<T: PolarsDataType> ChunkedArray<T> {
+    fn should_rechunk(&self) -> bool {
+        self.chunks.len() > 1 && self.chunks.len() > self.len() / 3
+    }
+
+    fn optional_rechunk(self) -> Self {
+        // Rechunk if we have many small chunks.
+        if self.should_rechunk() {
+            self.rechunk()
+        } else {
+            self
+        }
+    }
+
     /// Create a new [`ChunkedArray`] and compute its `length` and `null_count`.
     ///
     /// If you want to explicitly the `length` and `null_count`, look at
@@ -181,7 +194,7 @@ impl<T: PolarsDataType> ChunkedArray<T> {
 
     /// Get a reference to the used [`Metadata`]
     ///
-    /// This results a reference to an empty [`Metadata`] if its unset for this [`ChunkedArray`].
+    /// This results a reference to an empty [`Metadata`] if it's unset for this [`ChunkedArray`].
     #[inline(always)]
     pub fn effective_metadata(&self) -> &Metadata<T> {
         self.md.as_ref().map_or(&Metadata::DEFAULT, AsRef::as_ref)
diff --git a/crates/polars-expr/src/expressions/window.rs b/crates/polars-expr/src/expressions/window.rs
index 5a0169752c79..97f25abc4196 100644
--- a/crates/polars-expr/src/expressions/window.rs
+++ b/crates/polars-expr/src/expressions/window.rs
@@ -688,7 +688,7 @@ fn set_by_groups(
 
         macro_rules! dispatch {
             ($ca:expr) => {{
-                set_numeric($ca, groups, len)
+                Some(set_numeric($ca, groups, len))
             }};
         }
         downcast_as_macro_arg_physical!(&s, dispatch).map(|s| s.cast(dtype).unwrap())
@@ -697,7 +697,7 @@ fn set_by_groups(
     }
 }
 
-fn set_numeric<T>(ca: &ChunkedArray<T>, groups: &GroupsProxy, len: usize) -> Option<Series>
+fn set_numeric<T>(ca: &ChunkedArray<T>, groups: &GroupsProxy, len: usize) -> Series
 where
     T: PolarsNumericType,
     ChunkedArray<T>: IntoSeries,
@@ -709,10 +709,10 @@ where
     let sync_ptr_values = unsafe { SyncPtr::new(ptr) };
 
     if ca.null_count() == 0 {
+        let ca = ca.rechunk();
         match groups {
             GroupsProxy::Idx(groups) => {
-                // this should always succeed as we don't expect any chunks after an aggregation
-                let agg_vals = ca.cont_slice().ok()?;
+                let agg_vals = ca.cont_slice().expect("rechunked");
                 POOL.install(|| {
                     agg_vals
                         .par_iter()
@@ -727,8 +727,7 @@ where
                 })
             },
             GroupsProxy::Slice { groups, .. } => {
-                // this should always succeed as we don't expect any chunks after an aggregation
-                let agg_vals = ca.cont_slice().ok()?;
+                let agg_vals = ca.cont_slice().expect("rechunked");
                 POOL.install(|| {
                     agg_vals
                         .par_iter()
@@ -748,7 +747,7 @@ where
 
         // SAFETY: we have written all slots
         unsafe { values.set_len(len) }
-        Some(ChunkedArray::new_vec(ca.name(), values).into_series())
+        ChunkedArray::new_vec(ca.name(), values).into_series()
     } else {
         // We don't use a mutable bitmap as bits will have have race conditions!
         // A single byte might alias if we write from single threads.
@@ -826,6 +825,6 @@ where
             values.into(),
             Some(validity),
         );
-        Some(Series::try_from((ca.name(), arr.boxed())).unwrap())
+        Series::try_from((ca.name(), arr.boxed())).unwrap()
     }
 }
diff --git a/crates/polars/tests/it/lazy/functions.rs b/crates/polars/tests/it/lazy/functions.rs
index 8b4cc810d2ae..ff6dfec5a0b1 100644
--- a/crates/polars/tests/it/lazy/functions.rs
+++ b/crates/polars/tests/it/lazy/functions.rs
@@ -1,6 +1,7 @@
 use super::*;
 
 #[test]
+#[cfg(all(feature = "concat_str", feature = "strings"))]
 fn test_format_str() {
     let a = df![
         "a" => [1, 2],