Index: source/ps/GameSetup/GameSetup.h
===================================================================
--- source/ps/GameSetup/GameSetup.h
+++ source/ps/GameSetup/GameSetup.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2017 Wildfire Games.
+/* Copyright (C) 2018 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -79,6 +79,11 @@
 class CmdLineArgs;
 class Paths;
 extern const std::vector<CStr>& GetMods(const CmdLineArgs& args, int flags);
+
+/**
+ * Mounts all files of the given mods in the global VFS.
+ * Make sure to call CacheEnabledModVersions after every call to this.
+ */
 extern void MountMods(const Paths& paths, const std::vector<CStr>& mods);
 /**
  * Returns true if successful, false if mods changed and restart_engine was called.
Index: source/ps/GameSetup/GameSetup.cpp
===================================================================
--- source/ps/GameSetup/GameSetup.cpp
+++ source/ps/GameSetup/GameSetup.cpp
@@ -80,6 +80,7 @@
 #include "scriptinterface/ScriptInterface.h"
 #include "scriptinterface/ScriptStats.h"
 #include "scriptinterface/ScriptConversions.h"
+#include "scriptinterface/ScriptRuntime.h"
 #include "simulation2/Simulation2.h"
 #include "lobby/IXmppClient.h"
 #include "soundmanager/scripting/JSInterface_Sound.h"
@@ -922,6 +923,8 @@
 	const int heapGrowthBytesGCTrigger = 20 * 1024 * 1024;
 	g_ScriptRuntime = ScriptInterface::CreateRuntime(shared_ptr<ScriptRuntime>(), runtimeSize, heapGrowthBytesGCTrigger);
 
+	Mod::CacheEnabledModVersions(g_ScriptRuntime);
+
 	// Special command-line mode to dump the entity schemas instead of running the game.
 	// (This must be done after loading VFS etc, but should be done before wasting time
 	// on anything else.)
Index: source/ps/Mod.h
===================================================================
--- source/ps/Mod.h
+++ source/ps/Mod.h
@@ -30,6 +30,14 @@
 	JS::Value GetAvailableMods(const ScriptInterface& scriptInterface);
 
 	/**
+	 * This reads the version numbers from the launched mods.
+	 * It caches the result, since the reading of zip files is slow and
+	 * JS pages can request the version numbers too often easily.
+	 * Make sure this is called after each MountMods call.
+	 */
+	void CacheEnabledModVersions(shared_ptr<ScriptRuntime> scriptRuntime);
+
+	/**
 	 * Get the loaded mods and their version.
 	 * "user" mod and "mod" mod are ignored as they are irrelevant for compatibility checks.
 	 *
Index: source/ps/Mod.cpp
===================================================================
--- source/ps/Mod.cpp
+++ source/ps/Mod.cpp
@@ -31,6 +31,8 @@
 
 std::vector<CStr> g_modsLoaded;
 
+std::vector<std::vector<CStr>> g_LoadedModVersions;
+
 CmdLineArgs g_args;
 
 JS::Value Mod::GetAvailableMods(const ScriptInterface& scriptInterface)
@@ -101,29 +103,38 @@
 	return JS::ObjectValue(*obj);
 }
 
-JS::Value Mod::GetLoadedModsWithVersions(const ScriptInterface& scriptInterface)
+void Mod::CacheEnabledModVersions(shared_ptr<ScriptRuntime> scriptRuntime)
 {
+	ScriptInterface scriptInterface("Engine", "CacheEnabledModVersions", scriptRuntime);
 	JSContext* cx = scriptInterface.GetContext();
 	JSAutoRequest rq(cx);
 
 	JS::RootedValue availableMods(cx, GetAvailableMods(scriptInterface));
 
-	JS::RootedValue ret(cx, JS::ObjectValue(*JS_NewArrayObject(cx, 0)));
+	g_LoadedModVersions.clear();
 
-	// Index of the created array
-	size_t j = 0;
-	for (size_t i = 0; i < g_modsLoaded.size(); ++i)
+	for (const CStr& mod : g_modsLoaded)
 	{
 		// Ignore user and mod mod as they are irrelevant for compatibility checks
-		if (g_modsLoaded[i] == "mod" || g_modsLoaded[i] == "user")
+		if (mod == "mod" || mod == "user")
 			continue;
+
 		CStr version;
 		JS::RootedValue modData(cx);
-		if (scriptInterface.GetProperty(availableMods, g_modsLoaded[i].c_str(), &modData))
+		if (scriptInterface.GetProperty(availableMods, mod.c_str(), &modData))
 			scriptInterface.GetProperty(modData, "version", version);
-		scriptInterface.SetPropertyInt(ret, j++, std::vector<CStr>{g_modsLoaded[i], version});
+
+		g_LoadedModVersions.push_back({mod, version});
 	}
-	return ret;
+}
+
+JS::Value Mod::GetLoadedModsWithVersions(const ScriptInterface& scriptInterface)
+{
+	JSContext* cx = scriptInterface.GetContext();
+	JSAutoRequest rq(cx);
+	JS::RootedValue returnValue(cx);
+	scriptInterface.ToJSVal(cx, &returnValue, g_LoadedModVersions);
+	return returnValue;
 }
 
 JS::Value Mod::GetEngineInfo(const ScriptInterface& scriptInterface)
Index: source/ps/Replay.cpp
===================================================================
--- source/ps/Replay.cpp
+++ source/ps/Replay.cpp
@@ -179,6 +179,8 @@
 	const int heapGrowthBytesGCTrigger = 20 * 1024 * 1024;
 	g_ScriptRuntime = ScriptInterface::CreateRuntime(shared_ptr<ScriptRuntime>(), runtimeSize, heapGrowthBytesGCTrigger);
 
+	Mod::CacheEnabledModVersions(g_ScriptRuntime);
+
 	g_Game = new CGame(true, false);
 	if (serializationtest)
 		g_Game->GetSimulation2()->EnableSerializationTest();