Add a cache for lexers.Get

```
name                            old time/op    new time/op    delta
Codeblocks/Default-10              152ms ±11%      12ms ± 1%  -92.44%  (p=0.029 n=4+4)
Codeblocks/Hook_no_higlight-10     142ms ± 0%       7ms ± 0%  -95.36%  (p=0.029 n=4+4)

name                            old alloc/op   new alloc/op   delta
Codeblocks/Default-10             11.9MB ± 0%    11.7MB ± 0%   -1.59%  (p=0.029 n=4+4)
Codeblocks/Hook_no_higlight-10    4.62MB ± 1%    4.43MB ± 0%   -4.08%  (p=0.029 n=4+4)

name                            old allocs/op  new allocs/op  delta
Codeblocks/Default-10               209k ± 0%      209k ± 0%   -0.03%  (p=0.029 n=4+4)
Codeblocks/Hook_no_higlight-10     68.4k ± 0%     68.3k ± 0%   -0.06%  (p=0.029 n=4+4)

```

2 files changed, 52 insertions, 1 deletions

diff --git a/markup/highlight/chromalexers/chromalexers.go b/markup/highlight/chromalexers/chromalexers.go
new file mode 100644
index 000000000..41fd76261
--- /dev/null
+++ b/markup/highlight/chromalexers/chromalexers.go
@@ -0,0 +1,50 @@
+// Copyright 2022 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package chromalexers
+
+import (
+	"sync"
+
+	"github.com/alecthomas/chroma/v2"
+	"github.com/alecthomas/chroma/v2/lexers"
+)
+
+type lexersMap struct {
+	lexers map[string]chroma.Lexer
+	mu     sync.RWMutex
+}
+
+var lexerCache = &lexersMap{lexers: make(map[string]chroma.Lexer)}
+
+// Get returns a lexer for the given language name, nil if not found.
+// This is just a wrapper around chromalexers.Get that caches the result.
+// Reasoning for this is that chromalexers.Get is slow in the case where the lexer is not found,
+// which is a common case in Hugo.
+func Get(name string) chroma.Lexer {
+	lexerCache.mu.RLock()
+	lexer, found := lexerCache.lexers[name]
+	lexerCache.mu.RUnlock()
+
+	if found {
+		return lexer
+	}
+
+	lexer = lexers.Get(name)
+
+	lexerCache.mu.Lock()
+	lexerCache.lexers[name] = lexer
+	lexerCache.mu.Unlock()
+
+	return lexer
+}
diff --git a/markup/highlight/highlight.go b/markup/highlight/highlight.go
index 5b19d6e8e..010c941f7 100644
--- a/markup/highlight/highlight.go
+++ b/markup/highlight/highlight.go
@@ -28,6 +28,7 @@ import (
 	"github.com/gohugoio/hugo/common/text"
 	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/markup/converter/hooks"
+	"github.com/gohugoio/hugo/markup/highlight/chromalexers"
 	"github.com/gohugoio/hugo/markup/internal/attributes"
 )
 
@@ -167,7 +168,7 @@ func (h HightlightResult) Inner() template.HTML {
 func highlight(fw hugio.FlexiWriter, code, lang string, attributes []attributes.Attribute, cfg Config) (int, int, error) {
 	var lexer chroma.Lexer
 	if lang != "" {
-		lexer = lexers.Get(lang)
+		lexer = chromalexers.Get(lang)
 	}
 
 	if lexer == nil && (cfg.GuessSyntax && !cfg.NoHl) {