identity: Use xxHash in hashstructure (note)

```
name                 old time/op    new time/op    delta
HashString/n28-10       133ns ± 9%     107ns ±10%  -19.58%  (p=0.029 n=4+4)
HashString/n112-10      243ns ± 5%     121ns ± 4%  -50.08%  (p=0.029 n=4+4)
HashString/n448-10      698ns ± 3%     174ns ± 5%  -75.02%  (p=0.029 n=4+4)
HashString/n1792-10    2.58µs ± 4%    0.38µs ± 4%  -85.11%  (p=0.029 n=4+4)
HashString/n7168-10    10.0µs ± 3%     1.3µs ± 4%  -86.91%  (p=0.029 n=4+4)

name                 old alloc/op   new alloc/op   delta
HashString/n28-10       80.0B ± 0%     72.0B ± 0%  -10.00%  (p=0.029 n=4+4)
HashString/n112-10       160B ± 0%      152B ± 0%   -5.00%  (p=0.029 n=4+4)
HashString/n448-10       496B ± 0%      488B ± 0%   -1.61%  (p=0.029 n=4+4)
HashString/n1792-10    1.84kB ± 0%    1.83kB ± 0%   -0.43%  (p=0.029 n=4+4)
HashString/n7168-10    8.24kB ± 0%    8.23kB ± 0%   -0.06%  (p=0.029 n=4+4)

name                 old allocs/op  new allocs/op  delta
HashString/n28-10        4.00 ± 0%      3.00 ± 0%  -25.00%  (p=0.029 n=4+4)
HashString/n112-10       4.00 ± 0%      3.00 ± 0%  -25.00%  (p=0.029 n=4+4)
HashString/n448-10       4.00 ± 0%      3.00 ± 0%  -25.00%  (p=0.029 n=4+4)
HashString/n1792-10      4.00 ± 0%      3.00 ± 0%  -25.00%  (p=0.029 n=4+4)
HashString/n7168-10      4.00 ± 0%      3.00 ± 0%  -25.00%  (p=0.029 n=4+4)
```

8 files changed, 40 insertions, 18 deletions

diff --git a/config/namespace_test.go b/config/namespace_test.go
index 3bbb28259..5eacdeac7 100644
--- a/config/namespace_test.go
+++ b/config/namespace_test.go
@@ -43,7 +43,7 @@ func TestNamespace(t *testing.T) {
 	c.Assert(err, qt.IsNil)
 	c.Assert(ns, qt.Not(qt.IsNil))
 	c.Assert(ns.SourceStructure, qt.DeepEquals, map[string]interface{}{"foo": "bar"})
-	c.Assert(ns.SourceHash, qt.Equals, "5334326627423288605")
+	c.Assert(ns.SourceHash, qt.Equals, "1450430416588600409")
 	c.Assert(ns.Config, qt.DeepEquals, &tstNsExt{Foo: "bar"})
 	c.Assert(ns.Signature(), qt.DeepEquals, []*tstNsExt(nil))
 }
diff --git a/hugolib/image_test.go b/hugolib/image_test.go
index d58008512..637956b3b 100644
--- a/hugolib/image_test.go
+++ b/hugolib/image_test.go
@@ -82,12 +82,12 @@ SUNSET2: {{ $resized2.RelPermalink }}/{{ $resized2.Width }}/Lat: {{ $resized2.Ex
 	// Check the file cache
 	b.AssertImage(200, 200, "resources/_gen/images/bundle/sunset_hu59e56ffff1bc1d8d122b1403d34e039f_90587_200x200_resize_q75_box.jpg")
 
-	b.AssertFileContent("resources/_gen/images/bundle/sunset_9750822043026343402.json",
+	b.AssertFileContent("resources/_gen/images/bundle/sunset_2020904703097093419.json",
 		"FocalLengthIn35mmFormat|uint16", "PENTAX")
 
 	b.AssertImage(123, 234, "resources/_gen/images/images/sunset_hu59e56ffff1bc1d8d122b1403d34e039f_90587_123x234_resize_q75_box.jpg")
 
-	b.AssertFileContent("resources/_gen/images/images/sunset_9750822043026343402.json",
+	b.AssertFileContent("resources/_gen/images/images/sunset_2020904703097093419.json",
 		"FocalLengthIn35mmFormat|uint16", "PENTAX")
 
 	b.AssertNoDuplicateWrites()
diff --git a/identity/identityhash.go b/identity/identityhash.go
index 70371e567..7d3b83a0d 100644
--- a/identity/identityhash.go
+++ b/identity/identityhash.go
@@ -15,7 +15,9 @@ package identity
 
 import (
 	"strconv"
+	"sync"
 
+	"github.com/cespare/xxhash/v2"
 	"github.com/mitchellh/hashstructure/v2"
 )
 
@@ -28,6 +30,23 @@ func HashString(vs ...any) string {
 	return strconv.FormatUint(hash, 10)
 }
 
+var hashOptsPool = sync.Pool{
+	New: func() any {
+		return &hashstructure.HashOptions{
+			Hasher: xxhash.New(),
+		}
+	},
+}
+
+func getHashOpts() *hashstructure.HashOptions {
+	return hashOptsPool.Get().(*hashstructure.HashOptions)
+}
+
+func putHashOpts(opts *hashstructure.HashOptions) {
+	opts.Hasher.Reset()
+	hashOptsPool.Put(opts)
+}
+
 // HashUint64 returns a hash from the given elements.
 // It will panic if the hash cannot be calculated.
 // Note that this hash should be used primarily for identity, not for change detection as
@@ -44,7 +63,10 @@ func HashUint64(vs ...any) uint64 {
 		o = elements
 	}
 
-	hash, err := hashstructure.Hash(o, hashstructure.FormatV2, nil)
+	hashOpts := getHashOpts()
+	defer putHashOpts(hashOpts)
+
+	hash, err := hashstructure.Hash(o, hashstructure.FormatV2, hashOpts)
 	if err != nil {
 		panic(err)
 	}
diff --git a/identity/identityhash_test.go b/identity/identityhash_test.go
index 52debe293..fee38c8fe 100644
--- a/identity/identityhash_test.go
+++ b/identity/identityhash_test.go
@@ -25,12 +25,12 @@ import (
 func TestHashString(t *testing.T) {
 	c := qt.New(t)
 
-	c.Assert(HashString("a", "b"), qt.Equals, "2712570657419664240")
-	c.Assert(HashString("ab"), qt.Equals, "590647783936702392")
+	c.Assert(HashString("a", "b"), qt.Equals, "3176555414984061461")
+	c.Assert(HashString("ab"), qt.Equals, "7347350983217793633")
 
 	var vals []any = []any{"a", "b", tstKeyer{"c"}}
 
-	c.Assert(HashString(vals...), qt.Equals, "12599484872364427450")
+	c.Assert(HashString(vals...), qt.Equals, "4438730547989914315")
 	c.Assert(vals[2], qt.Equals, tstKeyer{"c"})
 }
 
diff --git a/resources/internal/key_test.go b/resources/internal/key_test.go
index b3f54eebd..fcad7d754 100644
--- a/resources/internal/key_test.go
+++ b/resources/internal/key_test.go
@@ -32,5 +32,5 @@ func TestResourceTransformationKey(t *testing.T) {
 	key := NewResourceTransformationKey("testing",
 		testStruct{Name: "test", V1: int64(10), V2: int32(20), V3: 30, V4: uint64(40)})
 	c := qt.New(t)
-	c.Assert(key.Value(), qt.Equals, "testing_5850874845946291698")
+	c.Assert(key.Value(), qt.Equals, "testing_4231238781487357822")
 }
diff --git a/resources/resource_factories/create/remote_test.go b/resources/resource_factories/create/remote_test.go
index 5284c4d19..293845107 100644
--- a/resources/resource_factories/create/remote_test.go
+++ b/resources/resource_factories/create/remote_test.go
@@ -127,10 +127,10 @@ func TestRemoteResourceKeys(t *testing.T) {
 		c.Assert(got2, qt.Equals, expect2)
 	}
 
-	check("foo", nil, "10276615683545312752", "10276615683545312752")
-	check("foo", map[string]any{"bar": "baz"}, "9898282343037894676", "9898282343037894676")
-	check("foo", map[string]any{"key": "1234", "bar": "baz"}, "14904296279238663669", "9898282343037894676")
-	check("foo", map[string]any{"key": "12345", "bar": "baz"}, "12191037851845371770", "9898282343037894676")
-	check("asdf", map[string]any{"key": "1234", "bar": "asdf"}, "14904296279238663669", "3005610248808565917")
-	check("asdf", map[string]any{"key": "12345", "bar": "asdf"}, "12191037851845371770", "3005610248808565917")
+	check("foo", nil, "7763396052142361238", "7763396052142361238")
+	check("foo", map[string]any{"bar": "baz"}, "5783339285578751849", "5783339285578751849")
+	check("foo", map[string]any{"key": "1234", "bar": "baz"}, "15578353952571222948", "5783339285578751849")
+	check("foo", map[string]any{"key": "12345", "bar": "baz"}, "14335752410685132726", "5783339285578751849")
+	check("asdf", map[string]any{"key": "1234", "bar": "asdf"}, "15578353952571222948", "15615023578599429261")
+	check("asdf", map[string]any{"key": "12345", "bar": "asdf"}, "14335752410685132726", "15615023578599429261")
 }
diff --git a/resources/resource_transformers/js/options_test.go b/resources/resource_transformers/js/options_test.go
index d2821c552..53aa9b6bb 100644
--- a/resources/resource_transformers/js/options_test.go
+++ b/resources/resource_transformers/js/options_test.go
@@ -44,7 +44,7 @@ func TestOptionKey(t *testing.T) {
 
 	key := (&buildTransformation{optsm: opts}).Key()
 
-	c.Assert(key.Value(), qt.Equals, "jsbuild_9458638411414195026")
+	c.Assert(key.Value(), qt.Equals, "jsbuild_1533819657654811600")
 }
 
 func TestToBuildOptions(t *testing.T) {
diff --git a/resources/resources_integration_test.go b/resources/resources_integration_test.go
index df489c9e5..9c511332f 100644
--- a/resources/resources_integration_test.go
+++ b/resources/resources_integration_test.go
@@ -159,9 +159,9 @@ resize 2|RelPermalink: {{ $image.RelPermalink }}|MediaType: {{ $image.MediaType
 	b := hugolib.Test(t, files)
 
 	b.AssertFileContent("public/index.html",
-		"jpg|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_14347832029652749901.jpg|MediaType: image/jpeg|Width: 1|Height: 1|",
-		"resize 1|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_14038289290831899195.jpg|MediaType: image/jpeg|Width: 20|Height: 30|",
-		"resize 2|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_14038289290831899195.jpg|MediaType: image/jpeg|Width: 20|Height: 30|",
+		"jpg|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_1397118720664523257.jpg|MediaType: image/jpeg|Width: 1|Height: 1|",
+		"resize 1|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_3796633496882814163.jpg|MediaType: image/jpeg|Width: 20|Height: 30|",
+		"resize 2|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_3796633496882814163.jpg|MediaType: image/jpeg|Width: 20|Height: 30|",
 	)
 }