diff --git a/core/src/ui/render_state.rs b/core/src/ui/render_state.rs
index 7b87280..d70d994 100644
--- a/core/src/ui/render_state.rs
+++ b/core/src/ui/render_state.rs
@@ -1090,7 +1090,13 @@ impl RegionRemap {
         if from.len() == to.len() || extent == Rel::ZERO {
             return scalar + to.start - from.start;
         }
-        let fraction = (scalar.rel - from.start.rel) / extent;
+        // A box that spans the whole of its parent's is the common one, and
+        // dividing by one is the expensive way to write a subtraction.
+        let offset = scalar.rel - from.start.rel;
+        let fraction = match extent == Rel::ONE {
+            true => offset,
+            false => offset / extent,
+        };
         let from_px = fraction.lerp(from.start.px, from.end.px);
         let to_rel = fraction.lerp(to.start.rel, to.end.rel);
         let to_px = fraction.lerp(to.start.px, to.end.px);