diff --git a/CHANGELOG.md b/CHANGELOG.md
index aae973d8ed..8c88a317e8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,9 +1,4 @@
-## 0.18.27-dev5
-
-### Enhancement
-- Upgrade pdfminer-six to 20260107 to fix ~15-18% performance regression from eager f-string evaluation
-
-## 0.18.27-dev4
+## 0.18.27-dev6
 
 ### Fixes
 - Comment no-ops in `zoom_image` (codeflash)
@@ -13,6 +8,8 @@
 - Optimize `_PartitionerLoader._load_partitioner` (codeflash)
 - Optimize `detect_languages` (codeflash)
 - Optimize `contains_verb` (codeflash)
+- Optimize `get_bbox_thickness` (codeflash)
+- Upgrade pdfminer-six to 20260107 to fix ~15-18% performance regression from eager f-string evaluation
 
 ## 0.18.26
 
diff --git a/unstructured/__version__.py b/unstructured/__version__.py
index c72d9a6020..c0da63dd46 100644
--- a/unstructured/__version__.py
+++ b/unstructured/__version__.py
@@ -1 +1 @@
-__version__ = "0.18.27-dev5"  # pragma: no cover
+__version__ = "0.18.27-dev6"  # pragma: no cover
diff --git a/unstructured/partition/pdf_image/analysis/bbox_visualisation.py b/unstructured/partition/pdf_image/analysis/bbox_visualisation.py
index 4de4828122..197a07f331 100644
--- a/unstructured/partition/pdf_image/analysis/bbox_visualisation.py
+++ b/unstructured/partition/pdf_image/analysis/bbox_visualisation.py
@@ -117,8 +117,9 @@ def _get_optimal_value_for_bbox(
         The optimal value for the given bounding box and parameters given.
     """
     bbox_to_page_ratio = _get_bbox_to_page_ratio(bbox, page_size)
-    coefficients = np.polyfit((ratio_for_min_value, ratio_for_max_value), (min_value, max_value), 1)
-    value = int(bbox_to_page_ratio * coefficients[0] + coefficients[1])
+    # Direct linear interpolation instead of np.polyfit for better performance
+    slope = (max_value - min_value) / (ratio_for_max_value - ratio_for_min_value)
+    value = int(min_value + slope * (bbox_to_page_ratio - ratio_for_min_value))
     return max(min_value, min(max_value, value))