Index: source/lib/path.h
===================================================================
--- source/lib/path.h
+++ source/lib/path.h
@@ -37,10 +37,14 @@
 #ifndef INCLUDED_PATH
 #define INCLUDED_PATH
 
+#include "lib/sysdep/os.h"
 #include "lib/utf8.h"
 
 #include <algorithm>
 #include <cstring>
+#if OS_WIN
+#include <filesystem>
+#endif
 #include <functional>
 
 namespace ERR
@@ -126,6 +130,19 @@
 		return path.empty();
 	}
 
+	// TODO: This macro should be removed later when macOS supports std::filesystem.
+	// Currently it does in more recent SDKs, but it also causes a slowdown on
+	// OpenGL. See #6193.
+#if OS_WIN
+	/**
+	 * @returns a STL version of the path.
+	 */
+	std::filesystem::path fileSystemPath() const
+	{
+		return std::filesystem::path(path);
+	}
+#endif
+
 	const String& string() const
 	{
 		return path;
Index: source/ps/Mod.cpp
===================================================================
--- source/ps/Mod.cpp
+++ source/ps/Mod.cpp
@@ -22,6 +22,7 @@
 #include "i18n/L10n.h"
 #include "lib/file/file_system.h"
 #include "lib/file/vfs/vfs.h"
+#include "lib/sysdep/os.h"
 #include "lib/utf8.h"
 #include "ps/Filesystem.h"
 #include "ps/GameSetup/GameSetup.h"
@@ -35,6 +36,9 @@
 #include <algorithm>
 #include <boost/algorithm/string/split.hpp>
 #include <boost/algorithm/string/classification.hpp>
+#if OS_WIN
+#include <filesystem>
+#endif
 #include <fstream>
 #include <sstream>
 #include <unordered_map>
@@ -48,11 +52,14 @@
 
 bool LoadModJSON(const PIVFS& vfs, OsPath modsPath, OsPath mod, std::string& text)
 {
+#if OS_WIN
+	const std::filesystem::path modJsonPath = (modsPath / mod / L"mod.json").fileSystemPath();
+#else
+	const std::string modJsonPath = (modsPath / mod / L"mod.json").string8();
+#endif
 	// Attempt to open mod.json first.
-	std::ifstream modjson;
-	modjson.open((modsPath / mod / L"mod.json").string8());
-
-	if (!modjson.is_open())
+	std::ifstream modjson(modJsonPath);
+	if (!modjson)
 	{
 		modjson.close();
 
@@ -69,8 +76,8 @@
 		text = modinfo.GetAsString();
 
 		// Attempt to write the mod.json file so we'll take the fast path next time.
-		std::ofstream out_mod_json((modsPath / mod / L"mod.json").string8());
-		if (out_mod_json.good())
+		std::ofstream out_mod_json(modJsonPath);
+		if (out_mod_json)
 		{
 			out_mod_json << text;
 			out_mod_json.close();