diff --git a/src/tests/transformers/shorthandCss.test.ts b/src/tests/transformers/shorthandCss.test.ts
index 9c8df424..884fa02d 100644
--- a/src/tests/transformers/shorthandCss.test.ts
+++ b/src/tests/transformers/shorthandCss.test.ts
@@ -1,6 +1,22 @@
-import { describe, it, expect } from 'vitest'
+import { describe, it, expect, vi } from 'vitest'
 import { shorthandCss, type ShorthandCssOptions } from '../../transformers/shorthandCss.ts'
 
+/**
+ * Count postcss runs: the transformer builds a fresh postcss
+ * processor per style value it merges, which calls this
+ * plugin creator once.
+ */
+const mergeLonghandCalls = vi.hoisted(() => ({ count: 0 }))
+
+vi.mock('postcss-merge-longhand', async (importOriginal) => {
+  const { default: plugin } = await importOriginal<typeof import('postcss-merge-longhand')>()
+  const counted = Object.assign((...args: Parameters<typeof plugin>) => {
+    mergeLonghandCalls.count++
+    return plugin(...args)
+  }, { postcss: true as const })
+  return { default: counted }
+})
+
 function run(html: string, options?: ShorthandCssOptions | boolean): string {
   if (options === false) return html
   const opts = (options === true || options == null) ? {} : options
@@ -18,6 +34,33 @@ describe('shorthandCss', () => {
       expect(result).not.toContain('margin-left:')
       expect(result).not.toContain('margin-right:')
     })
+
+    it('merges repeated identical style values on every element', () => {
+      const style = 'padding-top: 4px; padding-right: 8px; padding-bottom: 4px; padding-left: 8px'
+      const html = `<p style="${style}">A</p><td style="${style}">B</td><!--[if mso]><td style="${style}"><![endif]-->`
+      const result = shorthandCss(html)
+      expect(result.match(/padding: 4px 8px/g)).toHaveLength(3)
+      expect(result).not.toContain('padding-top:')
+    })
+
+    it('runs postcss once per distinct style value', () => {
+      const a = 'padding-top: 4px; padding-right: 8px; padding-bottom: 4px; padding-left: 8px'
+      const b = 'margin-top: 2px; margin-right: 2px; margin-bottom: 2px; margin-left: 2px'
+      const html = `<p style="${a}">A</p><td style="${a}">B</td><div style="${b}">C</div><!--[if mso]><td style="${a}"><![endif]-->`
+
+      mergeLonghandCalls.count = 0
+      shorthandCss(html)
+      expect(mergeLonghandCalls.count).toBe(2)
+    })
+
+    it('does not reuse cached style values across calls', () => {
+      const html = '<p style="padding-top: 4px; padding-right: 8px; padding-bottom: 4px; padding-left: 8px">A</p>'
+
+      mergeLonghandCalls.count = 0
+      shorthandCss(html)
+      shorthandCss(html)
+      expect(mergeLonghandCalls.count).toBe(2)
+    })
   })
 
   describe('margin shorthand', () => {
diff --git a/src/transformers/shorthandCss.ts b/src/transformers/shorthandCss.ts
index e8e39612..8f44dae2 100644
--- a/src/transformers/shorthandCss.ts
+++ b/src/transformers/shorthandCss.ts
@@ -99,25 +99,34 @@ export function shorthandCssDom(dom: ChildNode[], options: ShorthandCssOptions =
   const allowedTags = options.tags ?? []
   const hasTagFilter = allowedTags.length > 0
 
+  /**
+   * Utility-generated inline styles repeat heavily across elements, so
+   * each distinct value only goes through postcss once per call.
+   */
+  const cache = new Map<string, string>()
+
   /**
    * Merge longhand within a single inline-style value. Returns the merged
    * string when shorter, otherwise the original. Wraps the value in a
    * dummy selector since postcss-merge-longhand operates on rules.
    */
   const mergeStyleValue = (styleValue: string): string => {
+    let result = cache.get(styleValue)
+    if (result !== undefined) return result
+
+    result = styleValue
     try {
       const { css } = postcss()
         .use(mergeLonghand)
         .use(mergeBorder)
         .process(`div { ${styleValue} }`, { parser: safeParser })
       const match = css.match(/div\s*\{\s*([^}]+)\s*\}/)
-      if (match && match[1]) {
-        const merged = match[1].trim()
-        if (merged !== styleValue) return merged
-      }
+      if (match && match[1]) result = match[1].trim()
     }
     catch {}
-    return styleValue
+
+    cache.set(styleValue, result)
+    return result
   }
 
   walk(dom, (node) => {