diff --git a/Makefile b/Makefile
index 94bcd75..0ec69ba 100644
--- a/Makefile
+++ b/Makefile
@@ -4,6 +4,10 @@ COMPANION_WIN := companion/datascape-companion-windows-amd64.exe
 COMPANION_LIN := companion/datascape-companion-linux-amd64
 COMPANION_SRCS := $(wildcard cmd/companion/*.go) $(wildcard cmd/companion/*.html) go.mod go.sum
 
+# Release builds drop the symbol table, DWARF debug info and local source
+# paths. Panics still print file:line stack traces; only debuggers lose out.
+RELEASE_FLAGS := -trimpath -ldflags="-s -w"
+
 EDITOR_BUNDLE := assets/editor/vendor/codemirror.bundle.js
 EDITOR_SRCS := $(wildcard editor-build/*.js) editor-build/package.json editor-build/package-lock.json
 
@@ -14,10 +18,10 @@ EDITOR_SRCS := $(wildcard editor-build/*.js) editor-build/package.json editor-bu
 companion-release: $(COMPANION_WIN) $(COMPANION_LIN)
 
 $(COMPANION_WIN): $(COMPANION_SRCS)
-	GOOS=windows GOARCH=amd64 go build -ldflags="-H windowsgui" -o $@ ./cmd/companion
+	GOOS=windows GOARCH=amd64 go build -trimpath -ldflags="-s -w -H windowsgui" -o $@ ./cmd/companion
 
 $(COMPANION_LIN): $(COMPANION_SRCS)
-	GOOS=linux GOARCH=amd64 go build -o $@ ./cmd/companion
+	GOOS=linux GOARCH=amd64 go build $(RELEASE_FLAGS) -o $@ ./cmd/companion
 
 companion-windows: $(COMPANION_WIN)
 companion-linux: $(COMPANION_LIN)
@@ -37,7 +41,7 @@ $(EDITOR_BUNDLE): $(EDITOR_SRCS)
 	cd editor-build && npm ci && npm run build
 
 deploy: companion-release
-	GOOS=linux GOARCH=arm GOARM=7 go build -o datascape-arm .
+	GOOS=linux GOARCH=arm GOARM=7 go build $(RELEASE_FLAGS) -o datascape-arm .
 	ssh $(NAS) 'kill $$(cat /share/homes/luxick/.local/bin/datascape.pid) 2>/dev/null; rm -f /share/homes/luxick/.local/bin/datascape.pid'
 	scp datascape-arm $(NAS):/share/homes/luxick/.local/bin/datascape
 	ssh $(NAS) /share/homes/luxick/.local/bin/datascape-start.sh
diff --git a/README.md b/README.md
index b3839cf..7f19f45 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@ The wiki binary embeds the companion binaries, so build those first:
 ```bash
 make companion-release                         # once, and after companion changes
 go build -o datascape .                        # local
-GOOS=linux GOARCH=arm go build -o datascape .  # QNAP NAS
+make deploy                                    # QNAP NAS: stripped ARMv7 build, copy, restart
 ./datascape -dir ./wiki -addr :8080 -user me -pass secret
 ```
 
diff --git a/thumb_image.go b/thumb_image.go
index 30e0643..8f7dce8 100644
--- a/thumb_image.go
+++ b/thumb_image.go
@@ -57,6 +57,14 @@ func resizeBox(src image.Image, width int) image.Image {
 	}
 	dst := image.NewRGBA(image.Rect(0, 0, dstW, dstH))
 
+	// JPEGs decode to *image.YCbCr. Reading its planes directly avoids an
+	// interface call and a colour conversion per source pixel, which makes the
+	// resize ~4x faster; everything else takes the generic At() path.
+	avg := boxAvg
+	if ycc, ok := src.(*image.YCbCr); ok {
+		avg = func(_ image.Image, r image.Rectangle) color.RGBA { return boxAvgYCbCr(ycc, r) }
+	}
+
 	for y := 0; y < dstH; y++ {
 		sy0 := y * srcH / dstH
 		sy1 := (y + 1) * srcH / dstH
@@ -69,23 +77,42 @@ func resizeBox(src image.Image, width int) image.Image {
 			if sx1 == sx0 {
 				sx1 = sx0 + 1
 			}
-			var r, g, bl, n uint64
-			for sy := sy0; sy < sy1; sy++ {
-				for sx := sx0; sx < sx1; sx++ {
-					sr, sg, sb, _ := src.At(b.Min.X+sx, b.Min.Y+sy).RGBA()
-					r += uint64(sr >> 8)
-					g += uint64(sg >> 8)
-					bl += uint64(sb >> 8)
-					n++
-				}
-			}
-			dst.SetRGBA(x, y, color.RGBA{
-				R: uint8(r / n),
-				G: uint8(g / n),
-				B: uint8(bl / n),
-				A: 255,
-			})
+			box := image.Rect(sx0, sy0, sx1, sy1).Add(b.Min)
+			dst.SetRGBA(x, y, avg(src, box))
 		}
 	}
 	return dst
 }
+
+// boxAvg returns the mean colour of the pixels of src inside r.
+func boxAvg(src image.Image, r image.Rectangle) color.RGBA {
+	var red, g, bl, n uint64
+	for y := r.Min.Y; y < r.Max.Y; y++ {
+		for x := r.Min.X; x < r.Max.X; x++ {
+			sr, sg, sb, _ := src.At(x, y).RGBA()
+			red += uint64(sr >> 8)
+			g += uint64(sg >> 8)
+			bl += uint64(sb >> 8)
+			n++
+		}
+	}
+	return color.RGBA{R: uint8(red / n), G: uint8(g / n), B: uint8(bl / n), A: 255}
+}
+
+// boxAvgYCbCr is boxAvg for *image.YCbCr, reading the sample planes directly.
+// Each pixel is still converted to RGB before averaging so out-of-gamut
+// samples clamp exactly as they do through At().
+func boxAvgYCbCr(src *image.YCbCr, r image.Rectangle) color.RGBA {
+	var red, g, bl, n uint64
+	for y := r.Min.Y; y < r.Max.Y; y++ {
+		for x := r.Min.X; x < r.Max.X; x++ {
+			ci := src.COffset(x, y)
+			sr, sg, sb := color.YCbCrToRGB(src.Y[src.YOffset(x, y)], src.Cb[ci], src.Cr[ci])
+			red += uint64(sr)
+			g += uint64(sg)
+			bl += uint64(sb)
+			n++
+		}
+	}
+	return color.RGBA{R: uint8(red / n), G: uint8(g / n), B: uint8(bl / n), A: 255}
+}
diff --git a/thumb_image_test.go b/thumb_image_test.go
new file mode 100644
index 0000000..20a3fe8
--- /dev/null
+++ b/thumb_image_test.go
@@ -0,0 +1,56 @@
+package main
+
+import (
+	"bytes"
+	"image"
+	"image/jpeg"
+	"testing"
+)
+
+// testPhoto returns a JPEG-decoded (so *image.YCbCr, 4:2:0) image with an odd
+// size and a non-zero origin to exercise the plane offset arithmetic.
+func testPhoto(t testing.TB, w, h int) *image.YCbCr {
+	t.Helper()
+	src := image.NewRGBA(image.Rect(0, 0, w, h))
+	for i := range src.Pix {
+		src.Pix[i] = uint8(i*7 + i/w)
+	}
+	var buf bytes.Buffer
+	if err := jpeg.Encode(&buf, src, &jpeg.Options{Quality: 90}); err != nil {
+		t.Fatal(err)
+	}
+	img, err := jpeg.Decode(&buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return img.(*image.YCbCr).SubImage(image.Rect(3, 5, w, h)).(*image.YCbCr)
+}
+
+func TestResizeBoxYCbCrMatchesGeneric(t *testing.T) {
+	src := testPhoto(t, 1001, 667)
+	fast := resizeBox(src, 120).(*image.RGBA)
+	// Hide the concrete type so resizeBox takes the generic At() path.
+	slow := resizeBox(struct{ image.Image }{src}, 120).(*image.RGBA)
+	if fast.Bounds() != slow.Bounds() {
+		t.Fatalf("bounds %v != %v", fast.Bounds(), slow.Bounds())
+	}
+	for i := range fast.Pix {
+		if fast.Pix[i] != slow.Pix[i] {
+			t.Fatalf("pixel byte %d: fast %d, generic %d", i, fast.Pix[i], slow.Pix[i])
+		}
+	}
+}
+
+func BenchmarkResizeBox(b *testing.B) {
+	src := testPhoto(b, 4000, 3000)
+	b.Run("ycbcr", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			resizeBox(src, 300)
+		}
+	})
+	b.Run("generic", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			resizeBox(struct{ image.Image }{src}, 300)
+		}
+	})
+}