diff --git a/src/mime/multipart/formdata.go b/src/mime/multipart/formdata.go
index 41bc886..69b8b03 100644
--- a/src/mime/multipart/formdata.go
+++ b/src/mime/multipart/formdata.go
@@ -85,6 +85,7 @@
 			maxMemoryBytes = math.MaxInt64
 		}
 	}
+	var copyBuf []byte
 	for {
 		p, err := r.nextPart(false, maxMemoryBytes)
 		if err == io.EOF {
@@ -148,14 +149,22 @@
 				}
 			}
 			numDiskFiles++
-			size, err := io.Copy(file, io.MultiReader(&b, p))
+			if _, err := file.Write(b.Bytes()); err != nil {
+				return nil, err
+			}
+			if copyBuf == nil {
+				copyBuf = make([]byte, 32*1024) // same buffer size as io.Copy uses
+			}
+			// os.File.ReadFrom will allocate its own copy buffer if we let io.Copy use it.
+			type writerOnly struct{ io.Writer }
+			remainingSize, err := io.CopyBuffer(writerOnly{file}, p, copyBuf)
 			if err != nil {
 				return nil, err
 			}
 			fh.tmpfile = file.Name()
-			fh.Size = size
+			fh.Size = int64(b.Len()) + remainingSize
 			fh.tmpoff = fileOff
-			fileOff += size
+			fileOff += fh.Size
 			if !combineFiles {
 				if err := file.Close(); err != nil {
 					return nil, err
diff --git a/src/mime/multipart/formdata_test.go b/src/mime/multipart/formdata_test.go
index 8a862be..60f55b4 100644
--- a/src/mime/multipart/formdata_test.go
+++ b/src/mime/multipart/formdata_test.go
@@ -368,3 +368,52 @@
 		t.Fatalf("temp dir contains %v files; want 0", len(names))
 	}
 }
+
+func BenchmarkReadForm(b *testing.B) {
+	for _, test := range []struct {
+		name string
+		form func(fw *Writer, count int)
+	}{{
+		name: "fields",
+		form: func(fw *Writer, count int) {
+			for i := 0; i < count; i++ {
+				w, _ := fw.CreateFormField(fmt.Sprintf("field%v", i))
+				fmt.Fprintf(w, "value %v", i)
+			}
+		},
+	}, {
+		name: "files",
+		form: func(fw *Writer, count int) {
+			for i := 0; i < count; i++ {
+				w, _ := fw.CreateFormFile(fmt.Sprintf("field%v", i), fmt.Sprintf("file%v", i))
+				fmt.Fprintf(w, "value %v", i)
+			}
+		},
+	}} {
+		b.Run(test.name, func(b *testing.B) {
+			for _, maxMemory := range []int64{
+				0,
+				1 << 20,
+			} {
+				var buf bytes.Buffer
+				fw := NewWriter(&buf)
+				test.form(fw, 10)
+				if err := fw.Close(); err != nil {
+					b.Fatal(err)
+				}
+				b.Run(fmt.Sprintf("maxMemory=%v", maxMemory), func(b *testing.B) {
+					b.ReportAllocs()
+					for i := 0; i < b.N; i++ {
+						fr := NewReader(bytes.NewReader(buf.Bytes()), fw.Boundary())
+						form, err := fr.ReadForm(maxMemory)
+						if err != nil {
+							b.Fatal(err)
+						}
+						form.RemoveAll()
+					}
+
+				})
+			}
+		})
+	}
+}