diff --git a/font/renderer.go b/font/renderer.go
index 7c864db7..553209b0 100644
--- a/font/renderer.go
+++ b/font/renderer.go
@@ -3,11 +3,8 @@
 package font
 
 import (
-	"bytes"
-	"compress/gzip"
 	"errors"
 	"fmt"
-	"io"
 
 	ot "github.com/go-text/typesetting/font/opentype"
 	"github.com/go-text/typesetting/font/opentype/tables"
@@ -62,7 +59,8 @@ type GlyphSVG struct {
 	// The SVG image content, decompressed if needed.
 	// The actual glyph description is an SVG element
 	// with id="glyph<GID>" (as in id="glyph12"),
-	// and several glyphs may share the same Source
+	// and several glyphs may share the same Source.
+	// Source is read-only; copy it before modifying it.
 	Source []byte
 
 	// ViewBox is the initial viewport of the SVG document:
@@ -187,26 +185,15 @@ func (bt bitmap) glyphData(gid gID, xPpem, yPpem uint16) (GlyphBitmap, error) {
 }
 
 func (s svg) glyphData(gid gID, upem uint16) (GlyphSVG, bool) {
-	data, ok := s.rawGlyphData(gid)
+	doc, ok := s.rawGlyphData(gid)
 	if !ok {
 		return GlyphSVG{}, false
 	}
-
-	// un-compress if needed
-	if len(data) >= 2 && data[0] == 0x1f && data[1] == 0x8b {
-		r, err := gzip.NewReader(bytes.NewReader(data))
-		if err != nil {
-			return GlyphSVG{}, false
-		}
-		defer r.Close()
-		var buf bytes.Buffer
-		if _, err := io.Copy(&buf, r); err != nil {
-			return GlyphSVG{}, false
-		}
-		data = buf.Bytes()
+	data, viewBox, ok := doc.resolve(upem)
+	if !ok {
+		return GlyphSVG{}, false
 	}
-
-	return GlyphSVG{Source: data, ViewBox: svgViewBox(data, upem)}, true
+	return GlyphSVG{Source: data, ViewBox: viewBox}, true
 }
 
 // this file converts from font format for glyph outlines to
diff --git a/font/svg.go b/font/svg.go
index fca82f7c..89859260 100644
--- a/font/svg.go
+++ b/font/svg.go
@@ -4,11 +4,14 @@ package font
 
 import (
 	"bytes"
+	"compress/gzip"
 	"encoding/xml"
 	"fmt"
+	"io"
 	"math"
 	"strconv"
 	"strings"
+	"sync"
 
 	"github.com/go-text/typesetting/font/opentype/tables"
 )
@@ -39,20 +42,47 @@ type svgDocument struct {
 	svg   []byte
 	first gID // The first glyph ID in the range described by this index entry.
 	last  gID // The last glyph ID in the range described by this index entry. Must be >= startGlyphID.
+
+	// resolved on first use and shared by every glyph of the document
+	once     sync.Once
+	resolved []byte // un-compressed
+	viewBox  SVGViewBox
+	valid    bool
+}
+
+// resolve un-compresses the document if needed and parses its viewBox
+func (doc *svgDocument) resolve(upem uint16) ([]byte, SVGViewBox, bool) {
+	doc.once.Do(func() {
+		data := doc.svg
+		if len(data) >= 2 && data[0] == 0x1f && data[1] == 0x8b {
+			r, err := gzip.NewReader(bytes.NewReader(data))
+			if err != nil {
+				return
+			}
+			defer r.Close()
+			data, err = io.ReadAll(r)
+			if err != nil {
+				return
+			}
+		}
+		doc.resolved, doc.viewBox = data, svgViewBox(data, upem)
+		doc.valid = true
+	})
+	return doc.resolved, doc.viewBox, doc.valid
 }
 
 // rawGlyphData returns the SVG document for [gid], or false.
-func (s svg) rawGlyphData(gid gID) ([]byte, bool) {
+func (s svg) rawGlyphData(gid gID) (*svgDocument, bool) {
 	// binary search
 	for i, j := 0, len(s); i < j; {
 		h := i + (j-i)/2
-		entry := s[h]
+		entry := &s[h]
 		if gid < entry.first {
 			j = h
 		} else if entry.last < gid {
 			i = h + 1
 		} else {
-			return entry.svg, true
+			return entry, true
 		}
 	}
 	return nil, false
diff --git a/font/svg_test.go b/font/svg_test.go
index e05f7dbd..6faaed58 100644
--- a/font/svg_test.go
+++ b/font/svg_test.go
@@ -3,6 +3,9 @@
 package font
 
 import (
+	"bytes"
+	"compress/gzip"
+	"sync"
 	"testing"
 
 	"github.com/go-text/typesetting/font/opentype/tables"
@@ -77,3 +80,58 @@ func TestSVGOffsetOverflow(t *testing.T) {
 	}})
 	tu.Assert(t, err != nil)
 }
+
+func BenchmarkGlyphDataSVG(b *testing.B) {
+	font := loadFont(b, "toys/chromacheck-svg.ttf")
+	face := NewFace(font)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, _ = face.GlyphDataSVG(1)
+	}
+}
+
+func TestSVGResolveConcurrent(t *testing.T) {
+	const source = `<svg viewBox="1 2 30 40"/>`
+	var compressed bytes.Buffer
+	w := gzip.NewWriter(&compressed)
+	if _, err := w.Write([]byte(source)); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+	badChecksum := append([]byte(nil), compressed.Bytes()...)
+	badChecksum[len(badChecksum)-8] ^= 1
+	for _, test := range []struct {
+		name  string
+		data  []byte
+		valid bool
+	}{
+		{"plain", []byte(source), true},
+		{"gzip", compressed.Bytes(), true},
+		{"truncated header", []byte{0x1f, 0x8b}, false},
+		{"truncated body", compressed.Bytes()[:10], false},
+		{"invalid checksum", badChecksum, false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			s := svg{{first: 0, last: 7, svg: test.data}}
+			var wg sync.WaitGroup
+			for gid := gID(0); gid <= 7; gid++ {
+				wg.Add(1)
+				go func(gid gID) {
+					defer wg.Done()
+					for repeat := 0; repeat < 2; repeat++ {
+						glyph, ok := s.glyphData(gid, 1000)
+						if ok != test.valid {
+							t.Errorf("glyph %d: success %v, want %v", gid, ok, test.valid)
+						} else if ok && (string(glyph.Source) != source || glyph.ViewBox != (SVGViewBox{1, 2, 30, 40})) {
+							t.Errorf("glyph %d: unexpected source or viewBox: %+v", gid, glyph)
+						}
+					}
+				}(gid)
+			}
+			wg.Wait()
+		})
+	}
+}