diff --git a/internal/querylog/searchcriterion.go b/internal/querylog/searchcriterion.go
index c0aca44f..725a36fe 100644
--- a/internal/querylog/searchcriterion.go
+++ b/internal/querylog/searchcriterion.go
@@ -2,6 +2,8 @@ package querylog
 
 import (
 	"strings"
+	"unicode"
+	"unicode/utf8"
 
 	"github.com/AdguardTeam/AdGuardHome/internal/dnsfilter"
 )
@@ -63,6 +65,37 @@ func (c *searchCriterion) ctDomainOrClientCaseStrict(
 		strings.EqualFold(name, term)
 }
 
+// containsFold reports whehter s contains, ignoring letter case, substr.
+//
+// TODO(a.garipov): Move to aghstrings if needed elsewhere.
+func containsFold(s, substr string) (ok bool) {
+	sLen, substrLen := len(s), len(substr)
+	if sLen < substrLen {
+		return false
+	}
+
+	if sLen == substrLen {
+		return strings.EqualFold(s, substr)
+	}
+
+	first, _ := utf8.DecodeRuneInString(substr)
+	firstFolded := unicode.SimpleFold(first)
+
+	for i := 0; i != -1 && len(s) >= len(substr); {
+		if strings.EqualFold(s[:substrLen], substr) {
+			return true
+		}
+
+		i = strings.IndexFunc(s[1:], func(r rune) (eq bool) {
+			return r == first || r == firstFolded
+		})
+
+		s = s[1+i:]
+	}
+
+	return false
+}
+
 func (c *searchCriterion) ctDomainOrClientCaseNonStrict(
 	term string,
 	clientID string,
@@ -70,19 +103,10 @@ func (c *searchCriterion) ctDomainOrClientCaseNonStrict(
 	host string,
 	ip string,
 ) (ok bool) {
-	// TODO(a.garipov): Write a performant, case-insensitive version of
-	// strings.Contains instead of generating garbage.  Or, perhaps in the
-	// future, use a locale-appropriate matcher from golang.org/x/text.
-	clientID = strings.ToLower(clientID)
-	host = strings.ToLower(host)
-	ip = strings.ToLower(ip)
-	name = strings.ToLower(name)
-	term = strings.ToLower(term)
-
-	return strings.Contains(clientID, term) ||
-		strings.Contains(host, term) ||
-		strings.Contains(ip, term) ||
-		strings.Contains(name, term)
+	return containsFold(clientID, term) ||
+		containsFold(host, term) ||
+		containsFold(ip, term) ||
+		containsFold(name, term)
 }
 
 // quickMatch quickly checks if the line matches the given search criterion.
diff --git a/internal/querylog/searchcriterion_test.go b/internal/querylog/searchcriterion_test.go
new file mode 100644
index 00000000..65ee6645
--- /dev/null
+++ b/internal/querylog/searchcriterion_test.go
@@ -0,0 +1,121 @@
+package querylog
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestContainsFold(t *testing.T) {
+	testCases := []struct {
+		name     string
+		inS      string
+		inSubstr string
+		want     bool
+	}{{
+		name:     "empty",
+		inS:      "",
+		inSubstr: "",
+		want:     true,
+	}, {
+		name:     "shorter",
+		inS:      "a",
+		inSubstr: "abc",
+		want:     false,
+	}, {
+		name:     "same_len_true",
+		inS:      "abc",
+		inSubstr: "abc",
+		want:     true,
+	}, {
+		name:     "same_len_true_fold",
+		inS:      "abc",
+		inSubstr: "aBc",
+		want:     true,
+	}, {
+		name:     "same_len_false",
+		inS:      "abc",
+		inSubstr: "def",
+		want:     false,
+	}, {
+		name:     "longer_true",
+		inS:      "abcdedef",
+		inSubstr: "def",
+		want:     true,
+	}, {
+		name:     "longer_false",
+		inS:      "abcded",
+		inSubstr: "ghi",
+		want:     false,
+	}, {
+		name:     "longer_true_fold",
+		inS:      "abcdedef",
+		inSubstr: "dEf",
+		want:     true,
+	}, {
+		name:     "longer_false_fold",
+		inS:      "abcded",
+		inSubstr: "gHi",
+		want:     false,
+	}, {
+		name:     "longer_true_cyr_fold",
+		inS:      "абвгдедеё",
+		inSubstr: "дЕЁ",
+		want:     true,
+	}, {
+		name:     "longer_false_cyr_fold",
+		inS:      "абвгдедеё",
+		inSubstr: "жЗИ",
+		want:     false,
+	}, {
+		name:     "no_letters_true",
+		inS:      "1.2.3.4",
+		inSubstr: "2.3.4",
+		want:     true,
+	}, {
+		name:     "no_letters_false",
+		inS:      "1.2.3.4",
+		inSubstr: "2.3.5",
+		want:     false,
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			if tc.want {
+				assert.True(t, containsFold(tc.inS, tc.inSubstr))
+			} else {
+				assert.False(t, containsFold(tc.inS, tc.inSubstr))
+			}
+		})
+	}
+}
+
+var sink bool
+
+func BenchmarkContainsFold(b *testing.B) {
+	const s = "aaahBbBhccchDDDeEehFfFhGGGhHhh"
+	const substr = "HHH"
+
+	// Compare our implementation of containsFold against a stupid solution
+	// of calling strings.ToLower and strings.Contains.
+	b.Run("containsfold", func(b *testing.B) {
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			sink = containsFold(s, substr)
+		}
+
+		assert.True(b, sink)
+	})
+
+	b.Run("tolower_contains", func(b *testing.B) {
+		b.ReportAllocs()
+
+		for i := 0; i < b.N; i++ {
+			sink = strings.Contains(strings.ToLower(s), strings.ToLower(substr))
+		}
+
+		assert.True(b, sink)
+	})
+}