diff --git a/Package.swift b/Package.swift
index fe5447e8e..acc91a1be 100644
--- a/Package.swift
+++ b/Package.swift
@@ -4,6 +4,7 @@ import PackageDescription
 
 let package = Package(
     name: "XcodeProj",
+    platforms: [.macOS(.v11)],
     products: [
         .library(name: "XcodeProj", targets: ["XcodeProj"]),
     ],
diff --git a/Sources/XcodeProj/Utils/Collection+Extras.swift b/Sources/XcodeProj/Utils/Collection+Extras.swift
new file mode 100644
index 000000000..c332d2ce3
--- /dev/null
+++ b/Sources/XcodeProj/Utils/Collection+Extras.swift
@@ -0,0 +1,34 @@
+extension Collection where Element: BinaryInteger, Index == Int {
+    @inlinable
+    @inline(__always)
+    func containsCString<T: BidirectionalCollection>(_ cString: T) -> Bool where T.Element: BinaryInteger, T.Index == Int {
+        guard !cString.isEmpty else { return true }
+
+        // Drop null terminator if present
+        let subarrayCount = cString.last == 0
+            ? cString.count - 1
+            : cString.count
+
+        guard subarrayCount <= count else { return false }
+
+        let lastSubarrayStartingPos = count - subarrayCount
+        var i = 0
+        while i <= lastSubarrayStartingPos {
+            var match = true
+            var j = 0
+            while j < subarrayCount {
+                if self[i + j] != cString[j] {
+                    match = false
+                    break
+                }
+                j += 1
+            }
+            if match {
+                return true
+            }
+
+            i += 1
+        }
+        return false
+    }
+}
diff --git a/Sources/XcodeProj/Utils/CommentedString.swift b/Sources/XcodeProj/Utils/CommentedString.swift
index 2420f8e05..278214517 100644
--- a/Sources/XcodeProj/Utils/CommentedString.swift
+++ b/Sources/XcodeProj/Utils/CommentedString.swift
@@ -1,5 +1,31 @@
 import Foundation
 
+private extension UInt8 {
+    static let tab: UInt8 = 9 // '\t'
+    static let newline: UInt8 = 10 // '\n'
+    static let backslash: UInt8 = 92 // '\'
+    static let underscore: UInt8 = 95 // '_'
+    static let doubleQuotes: UInt8 = 34 // '"'
+    static let dollar: UInt8 = 36 // '$'
+    static let slash: UInt8 = 47 // '/'
+
+    static let dot: UInt8 = 46 // '.'
+    static let nine: UInt8 = 57 // '9'
+
+    static let capitalA: UInt8 = 65 // 'A'
+    static let capitalZ: UInt8 = 90 // 'Z'
+
+    static let smallA: UInt8 = 97 // 'a'
+    static let smallN: UInt8 = 110 // 'n'
+    static let smallT: UInt8 = 116 // 't'
+    static let smallZ: UInt8 = 122 // 'z'
+}
+
+private extension ContiguousArray<CChar> {
+    static let slashesUTF8CString = "//".utf8CString
+    static let threeUnderscoresUTF8CString = "___".utf8CString
+}
+
 /// String that includes a comment
 struct CommentedString {
     /// Entity string value.
@@ -18,19 +44,6 @@ struct CommentedString {
         self.comment = comment
     }
 
-    /// Set of characters that are invalid.
-    private static let invalidCharacters: CharacterSet = {
-        var invalidSet = CharacterSet(charactersIn: "_$")
-        invalidSet.insert(charactersIn: UnicodeScalar(".") ... UnicodeScalar("9"))
-        invalidSet.insert(charactersIn: UnicodeScalar("A") ... UnicodeScalar("Z"))
-        invalidSet.insert(charactersIn: UnicodeScalar("a") ... UnicodeScalar("z"))
-        invalidSet.invert()
-        return invalidSet
-    }()
-
-    /// Set of characters that are invalid.
-    private static let specialCheckCharacters = CharacterSet(charactersIn: "_/")
-
     /// Returns a valid string for Xcode projects.
     var validString: String {
         switch string {
@@ -40,31 +53,31 @@ struct CommentedString {
         default: break
         }
 
-        if string.rangeOfCharacter(from: CommentedString.invalidCharacters) == nil {
-            if string.rangeOfCharacter(from: CommentedString.specialCheckCharacters) == nil {
-                return string
-            } else if !string.contains("//"), !string.contains("___") {
-                return string
+        var str = string
+        return str.withUTF8 { buffer -> String in
+            let containsInvalidCharacters = buffer.containsInvalidCharacters
+
+            if !containsInvalidCharacters() {
+                let containsSpecialCheckCharacters = buffer.containsSpecialCheckCharacters()
+
+                if !containsSpecialCheckCharacters {
+                    return string
+                } else if !buffer.containsCString(ContiguousArray.slashesUTF8CString),
+                          !buffer.containsCString(ContiguousArray.threeUnderscoresUTF8CString) {
+                    return string
+                }
             }
-        }
 
-        let escaped = string.reduce(into: "") { escaped, character in
-            // As an optimization, only look at the first scalar. This means we're doing a numeric comparison instead
-            // of comparing arbitrary-length characters. This is safe because all our cases are a single scalar.
-            switch character.unicodeScalars.first {
-            case "\\":
-                escaped.append("\\\\")
-            case "\"":
-                escaped.append("\\\"")
-            case "\t":
-                escaped.append("\\t")
-            case "\n":
-                escaped.append("\\n")
-            default:
-                escaped.append(character)
+            // calculate exact size
+            let escapedCapacity = buffer.escapedCommentCapacity()
+
+            // write directly into String storage
+            return String(unsafeUninitializedCapacity: escapedCapacity) { stringBuffer in
+                stringBuffer.fillValidString(from: buffer)
+
+                return escapedCapacity
             }
         }
-        return "\"\(escaped)\""
     }
 }
 
@@ -95,3 +108,107 @@ extension CommentedString: ExpressibleByStringLiteral {
         self.init(value)
     }
 }
+
+// MARK: - Private
+
+private extension UnsafeMutableBufferPointer<UInt8> {
+    /// Fills preallocated `UnsafeBufferPointer<UInt8>`
+    func fillValidString(from buffer: UnsafeBufferPointer<UInt8>) {
+        var outIndex = 0
+
+        self[outIndex] = .doubleQuotes
+        outIndex += 1
+
+        for character in buffer {
+            switch character {
+            case .backslash:
+                self[outIndex] = .backslash
+                self[outIndex + 1] = .backslash
+                outIndex += 2
+
+            case .doubleQuotes:
+                self[outIndex] = .backslash
+                self[outIndex + 1] = .doubleQuotes
+                outIndex += 2
+
+            case .tab:
+                self[outIndex] = .backslash
+                self[outIndex + 1] = .smallT
+                outIndex += 2
+
+            case .newline:
+                self[outIndex] = .backslash
+                self[outIndex + 1] = .smallN
+                outIndex += 2
+
+            default:
+                self[outIndex] = character
+                outIndex += 1
+            }
+        }
+
+        self[outIndex] = .doubleQuotes
+    }
+}
+
+private extension UnsafeBufferPointer<UInt8> {
+    /// Valid characters are:
+    /// 1. `_` and `$`
+    /// 2. `.`...`9`
+    /// 3. `A`...`Z`
+    /// 4. `a`...`z`
+    func containsInvalidCharacters() -> Bool {
+        for character in self {
+            // character == '_' || character == '$'
+            if character == .underscore || character == .dollar {
+                continue
+            }
+            // character >= '.' && character <= '9'
+            if character >= .dot, character <= .nine {
+                continue
+            }
+            // character >= 'A' && character <= 'Z'
+            if character >= .capitalA, character <= .capitalZ {
+                continue
+            }
+            // character >= 'a' && character <= 'z'
+            if character >= .smallA, character <= .smallZ {
+                continue
+            }
+
+            return true
+        }
+
+        return false
+    }
+
+    /// Special check characters are `_` and `/`
+    func containsSpecialCheckCharacters() -> Bool {
+        for character in self {
+            if character == .underscore || character == .slash {
+                return true
+            }
+        }
+
+        return false
+    }
+
+    /// Calculates escaped string size
+    /// Basically, `count + count(where: { [.backslash, .doubleQuotes, .tab, .newline].contains($0) }`
+    func escapedCommentCapacity() -> Int {
+        var escapeCount = 0
+
+        for character in self {
+            switch character {
+            case .backslash, .doubleQuotes, .tab, .newline:
+                escapeCount += 1 // each adds one extra byte
+            default:
+                break
+            }
+        }
+
+        return count // original bytes
+            + escapeCount // extra escape bytes
+            + 2 // surrounding quotes
+    }
+}