diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index 205acad83dd..de22f1a427d 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -141,17 +141,13 @@ impl BlockContainer {
         containing_block_writing_mode: WritingMode,
     ) -> ContentSizes {
         match &self {
-            Self::BlockLevelBoxes(boxes) => {
-                let mut content_sizes = ContentSizes::zero();
-                for box_ in boxes {
-                    content_sizes.max_assign(
-                        &box_
-                            .borrow_mut()
-                            .inline_content_sizes(layout_context, containing_block_writing_mode),
-                    );
-                }
-                content_sizes
-            },
+            Self::BlockLevelBoxes(boxes) => boxes
+                .par_iter()
+                .map(|box_| {
+                    box_.borrow_mut()
+                        .inline_content_sizes(layout_context, containing_block_writing_mode)
+                })
+                .reduce(ContentSizes::zero, ContentSizes::max),
             Self::InlineFormattingContext(context) => {
                 context.inline_content_sizes(layout_context, containing_block_writing_mode)
             },
diff --git a/components/layout_2020/sizing.rs b/components/layout_2020/sizing.rs
index ec07861c374..ccf60ceb933 100644
--- a/components/layout_2020/sizing.rs
+++ b/components/layout_2020/sizing.rs
@@ -33,9 +33,11 @@ impl ContentSizes {
         }
     }
 
-    pub fn max_assign(&mut self, other: &Self) {
-        self.min_content.max_assign(other.min_content);
-        self.max_content.max_assign(other.max_content);
+    pub fn max(self, other: Self) -> Self {
+        Self {
+            min_content: self.min_content.max(other.min_content),
+            max_content: self.max_content.max(other.max_content),
+        }
     }
 
     /// Relevant to outer intrinsic inline sizes, for percentages from padding and margin.